Well obviously he wants to do it, and he obviously knows it doesn't hurt him when he doesn't, so by saying "don't do that" you're being patronizing. It's pretty insulting.

His situation is more of "When I bend my wrist backwards, it hurts". He's using it in a way that is not necessarily good. Yes, it is included. And yes, it may be helpful/beneficial. But if you're having problems with doing things that way, stop.