A lot of contracts do have clauses. It's why you always hear on the news that a baseball player broke their thumb "playing cards." A lot of the time, they aren't aloud to play any other physical contact sport, so they make up some bogus excuse so they don't get fined/cut. It's not the "law" that he can't ride a bike, so it's his own choice. He takes the risk of fines/being cut if that's in his contract.
Mark
