Michael Phelps means nothing to me, and if he wants to use marijuana, oh well. Stupid is as stupid does.
However, if the rules for competition are “no drug use ever,” then (if this is really he) he shouldn’t be allowed to compete any more. With no actual drug charges on his record, he should be able to get a real job.
But still, can you imagine how far he could have gotten as a competitive swimmer if he just hadn't smoked marijuana?