Impossible according to what? The laws of the universe that god made? Couldn't he just as easily have made different laws?
And why is the capability to commit evil always a defining characteristic of free will? We also cannot breathe underwater, fly, or lift a freight train with our god-given anatomy, so do we actually have free will?
Compare it to our own legal system: you have free will to choose what you want for dinner tonight, right? Loads of restaurants with loads of entrees, grocery stores stocked with whatever you need to cook up your creation: the combination of options is unfathomably vast. Free, right? ...but wait, according to the law, you're not allowed to cannibalize your neighbor's 4 year old kid, so I guess that makes you a mindless drone cuz if evil isn't on the menu then neither is free will.
So let's put that restriction back in god's court: he doesn't need to enforce his law, he just wills it and so it is: you still have the unfathomably huge number of non-evil options, but this time cannibalism is actually impossible: you are literally incapable of committing or even considering it because divine intervention will not let you - do you have free will?
Our freedom exists within the constraints of some system. Both legal and divine. There are loads of things we aren't free to do - the capacity to commit evil taken away from us would be a drop in the ocean.