In general, I think that guys getting chippy and trying to bully their opponent for an athletic edge is fair game. And I think a wrestler responding to a bully attempt is also fair game. Wrestling, after all, is a regulated fight.
But attempting to injure an opponent is totally unacceptable. I don't think MM attempted to injure JB, but if he pushed low on his legs while out of bounds, that would be a grey area IMO.