Oh no, Michael Phelps and the demon weed marijuana! He's going to lose all his endorsements and his motivation, besides! Relax, people. This can all work out to the advantage of his nerd-ass reputation.
First of all, let's dispense with the idea that Phelps has like totally blown his reputation and his future and all his role model endorsement monies. You know who thinks that? People like Andrea Peyser, the New York Post's own daily beast, who rounds up some branding experts to say Phelps has destroyed himself with this wanton bong-hitting. But how many Speedos is Andrea Peyser buying? Not many!
In fact, we interpret this as Phelps' attempt to break out of the Nerdy Endorsement Trap that we warned him about. He's been off hanging with Jared from Subway, which does nothing for his coolness quotient. So he's trying to establish his rock star-ness. Grabbing strippers! Trashing hotel rooms! And now, partaking of the ganja, straight from the grave of Solomon.
Of course he's not successfully breaking away from his nerdery, because he is, deep down, a nerd. But he's giving it the old college try. And if he can at least somewhat separate himself from this dreary, totally undeserved "All American Boy" reputation, he would get in far less trouble for smoking weed, which every last American, including athletes, has done anyhow, except for a few dead-enders who you would not want to model yourself after.
Finally: this guy is a swimmer. Nobody pays attention to swimmers except for when the Olympics are on, and a brief period of hype directly before and after the Olympics. So, way to keep yourself in the news, Mike. Do whatever you want until six months before the next summer Olympics, then act moderately well-behaved. You'll be fine. Try not to be a dick.