I'm surprised his NFL contract didn't stipulate that he not engage in dangerous off-the-field activities, such as riding motorcycles without appropriate protective equipment.
So was I.
Its up to each NFL team to insert that kind of language in there.
You'd be surprised at how restrictive baseball contracts are, alot of players are banned from playing even recreational basketball, Don Mattinly's contract, one of the few I have ever seen, pretty much banned everything you could possibly name, and listed a whole boatload of sports he was not allowed to play at anytime without permission.
The bargaining agreements allow teams alot of leeway in what restrictions they want to place on players.
NFL teams, are allowed to prohibit motorcycle use during the regular season unless the contract states otherwise.