Maybe because that is what the teacher is teaching and the student is not advanced to the point of knowing how to do free technique and reversals? Sometimes teachers don't want their students just doing whatever they feel is right because they want the student to work on something specific? Maybe the student does not yet have enough experience to be able to discern ,"what the situation calls for"?
If one wants to practice a specific technique and this is what is being taught it isn't really good form to be a resistant uke to the point of making your fellow students feel discouraged from trying to learn.That sort of thing is best left for two similar level students who have been around a bit and who have both agreed that that is the kind of practice they are up for.
Then a simple "Hey, I'm a noob and just learning, could you not crush my wrist quite so much please?" should suffice. But, maybe not, since the noob probably already said that when he or she bows to uke and says "O-negai shimasu." If muley uke is more advanced than nage, he or she should know this. If uke is not more advanced than nage, he or she should learn this.
Maybe sensei could also demonstrate what uke's role should be for the set practice technique.