Exactly, these athletes are role models and in the public eye. Drug testing is supposed to even the playing field and ensure there is no cheating or performance enhancing supplement use. Also, it ensures the athletes are clean. As marijuana is illegal in most countries it makes sense that it is a banned substance. This goes for all sports across the board.
As much as the community on here is "pro-weed" you can't have it being socially acceptable for these future Olympians to be toking up, it would never fly. Young children, even teens idiolize and look up to these athletes, and the the Olympic Motto is higher, faster, stronger. It's about athletic prowess, and ability. Where do drugs fit in to that?
Athletes in every other sport manage to be clean, still perform/compete and have fun. Why should halfpipe skiing be an exception to this? If advancement of their sport, and involvement in the Olympics is more important to them I'm sure they are capable of giving up marijuana for a few years. It just comes down to what's more important. Getting high or competing in the Olympics? They can always smoke up again once they retire, or go back to just the pro circuit and stop competing in FIS events.