And one possible answer is that no one who has been given that freedom ever actually does turn away from God. So the "choice" exists in principle but is never actually made. But there would seem to be some Pauline passages which would suggest that it's a real possibility, since he seems to expend some rhetorical energy urging people NOT to make that choice.
The choice is in obeying God or not. God will never lose one whom he calls a child. We can sin, and we often do, but we cannot lose the love of God once he has chosen us. The choice is to obey or not obey.