I don't see where denying the omniscience is even seen. I can see the logic in the allegation that the doctrine of free will denies omnipotence of God. But that logic is faulty because, in fact, it is the Calvinist notion that man cannot work outside of the will of God that denies God the power to create man who can do just that. In their worldview, the crown of the Creation, the jewel of the Sixth Day really isn't such big of a deal.
Here is an Orthodox take on this, with Biblical references and all. I am sure you will think tat it could have been written by a Latin bishop just the same. It clearly shows that we must be willing to accept God's offer. My take on it is much simpler: the Protestant notion of "robot believers" makes the concept of man's sin, fall, repentance and redemption totally meaningless.
Consider just one of these, say, repentance. In Greek the word is metanoia which means literally changing your mind [for the better]. If we have no free will, repentance becomes impossible.
Without free will, we cannot sin. Without sin, our Fall is cruel and unjust punishment. Without sin of our own, how can we be redeemed for something we never had?
If evil and sin is something God created, then why did He become Incarnate to die for our sins? Why bother, when the beginning and the end of this movie is already known -- good guys live, and bad guys perish no matter what.