As far as I know, it's not illegal in football (soccer) to kick the ball really hard at someone's face if they are in the way of goal. Throwing dummies and gamesmanship are also treated as acceptable.
So how exactly does agreeing on rules of a game remove normal moral constraints? I know people wouldn't be happy if I started blasting a football at their faces, but would it be morally ok?
It's not illegal but practitioners of the game would certainly be judged to be immoral if it was done with the intention of hurting someone. It is true though that we can do things in sports that would be judged to be immoral in other contexts and on this point I agree with Douglas Burnham that it is a matter of giving consent - accepting the rules of the game.