Krsna allows free will (voluntary self-expression) so His devotees can experience their own unique individuality by making their own offerings or contributions to Him in a way that forever expands loving exchanges and reciprocation in a two-way transition.

"Free will" eternally exists for Krsna's devotees because He wants a two-way relationship and therefore encourages self expression and personal contributions that make the relationship a two-way exchange of loving emotions.

In this way, Krsna allows the individual jiva-souls to continually re-invent themselves in the spiritual world, by eternally voluntarily making their own unique contributions and offerings, allowing diversity and the existence of a progressive independent free spirit who are always forever expanding their active participation, personal contributions and inspiration in an unlimited variety of unique ways serving Krsna.

Krsna allows the individual jiva-souls (His devotees) to have their free will because He wants to see unique independent expressions of love towards Him from His devotee's personal view point, with their own unique thoughtful contributing offerings without any influence from Krsna. 

Krsna wants His devotees to be personally creative in their independent offerings, friendship and service to Him.

In other words, Kṛṣṇa wants a "two-sided" open reciprocal cooperative contributing relationship of loving exchanges with His intimate devotees, where they voluntarily express themselves in their own unique way as separate individuals from Krsna.

The relationship with Krsna must be this way because love can never be experienced with just one, there must be an open "two-sided" reciprocal relationship. 

In this way, Krsna gave His devotees their freedom (free-will) so they can voluntarily choose to serve Kṛṣṇa in unlimited creative wsys, or even reject Him if they choose, this will only prove Krsna has allowed free will.

Ultimately this is all Krsna's creation and how He manages it is entirely up to Him as Bhagavad Gita As It Is explains.

However, the devotees having their freedom of self expression must be this way because Krsna knows if He never gave His devotees (the individual jīva-souls) their independence (free will), then genuine loving exchanges, affection and reciprocation in a open two-way exchange, could never exist, making Krsna no different from a cold puppet master who manipulates and controls everything his puppets do and think.

Such a mindless dictatorial existence without free will (unique self expression of voluntary offerings to Krsna) is dangerous impersonalism. 

Therefore, Krsna's devotees (individual jiva-souls) are eternal independent thinking thoughtful spiritual living PERSONS, as an eternal spiritual bodily form like Krsna's Bodily Form, who are indestructible and can never be destroyed, terminated, cut into pieces or extinguished as Bhagavad Gita As It Is chapter two explains.

On the Vaikuntha planets and in Goloka Vrindavana, personal voluntary expressions of loving exchanges expressed through free will, are always encouraged by Krsna in His eternal relationship with His devotees that adds flavour, variety and mystery to the association and intimate relationship.

This means the Kingdom of God is not a "one-sided" dominating impersonal domain devoid of free will (voluntary service and personal contributions) 

Such a "one-sided" dictatorial relationship does not allow personal contributions or offerings to God (Krsna) that are only found in a "two-way" relationship that expands, enrichs and flavours the devotees personal voluntary service to Krsna.

In an impersonal "one-sided" relationship, where a devotee is always told what to do, how to think and how to act, only denies the devotee's their individual God given ability of self expression that provides unique contributions and voluntary loving exchanges.

As said above, love or service is never a "one-sided" affiliate of total supremacy or preeminence in the spiritual world because for love to flourish and expand there MUST be two. 

Genuine loving relationships between Kṛṣṇa and His devotees are based on voluntary reciprocal exhanges and personal contributions between two, love can never be experienced by just one.

The devotees (individual jiva-souĺs) have their choices on how they want to serve and please Krsna too, or they can even choose to reject Krsna if they want, this is the consequence of having free will.

Loving reciprocation is only possible on a "two-way" street between two, with each contributing to the relationship's expansion of nectarine mellows.

The word "reciprocation" only has meaning when there are two involved, not one.

The individual jiva-souĺs (devotees) have their choices too, even if that sometimes means rejecting Krsna and doing their own thing in the temporary material world.

The individual devotees in their full potential on the Vaikuntha planets and Goloka Vrindavana, are forever positively re-inventing themselves by always choosing how to continuously increase their variety of devotional voluntary service to Krsna or Visnu.

This is achieved by forever positively discovering a variety of ways to expand their individual expressions of personal offerings and affection to Krsna in their perpetual unique relationship with Him.

Srila Prabhupada – “Love is reciprocal, voluntary, good exchange of feeling. Then there is love. Not by force, no, Krsna does not want to become a lover like that, ‘You love me, otherwise I shall kill you!” (Washington DC July 8, 1976)

As said above, real love (service) is voluntary and can never exist on a “one-way" impersonal path because love can only exists when "two" are involved in voluntary reciprocal contributing loving exchanges.

Srila Prabhupada – “The impersonalist philosophy is oneness so how there be love with just one? Is it possible? Have you got any such experience that love means one? No. Love means two. There must be two, the lover and the beloved. Kṛṣṇa is already lover. He loves you so much that He's trying to get you back. That is Kṛṣṇa's attempt. "Please, My dear boy, or My dear friend, My dear servant.” (Lecture SB, Canto 2 Ch 1 text 3, Paris, June 12, 1974)

Srila Prabhupada – “Love means you take and you give also. Suppose if you love somebody and you simply take from him, but you don't give. Oh, do you think it is very good? No. It is not good. That is not love. That is exploitation. If I go on simply taking from you, and if I don't offer you anything, that is simply exploitation.” (Lecture BG, Ch 9 text 2-5, New York City, Nov 23, 1966)

The relationship with Krsna in the spiritual world (The Kingdom of God) is never a “one-way” forced unproductive affinity (impersonalism).

The individual devotee's relationship with Krsna is always based on "free will, " this allows a variety of unique creative voluntary offerings of service to Krsna, only then can genuine loving exchanges exist.

Śrīla Prabhupāda – “Love means "two-persons," I must deal with you open-hearted, you must deal with me open-hearted, then there is love. So if you want to love Kṛṣṇa, then these things must be there.” (Aug 9, 1976, Tehran)

Personal voluntarily self chosen contributions offered by Krsna's devotees, are different from Krsna's shower of loving affection to His devotees, putting the relationship on a "two-way" street of loving exchanges.

The relationship between Krsna and His devotees are eternally a "two-way" mystery of exchanges that only further enriches, intensifies and expands loving expressions and affection between the devotees and Krsna.

Such personal unique contributions from the individual devotee's independent expressions, coming from their independent execution of free will, forever expands their personal ability to make voluntary offerings to Krsna in the spiritual world that forever increase their loving emotions for Krsna in a healthy "two-way" exchange of loving reciprocation.