Plenty of people smoke weed. In fact, people in Colorado are getting used to the idea that it's not even illegal anymore.
But when you're a professional athlete, you're not allowed to do it (unless you play hockey—woot!). At least during the season. Does that stop the biggest stars in the game? Of course not. But just don't be all that surprised when they get "randomly" drug-tested.
The league needs some way to make sure its players are clean, and hey, "random" drug testing does the trick. Teams know who their most frequent offenders are, and they probably have their ways of making sure those offenders get "randomly" tested fairly often.
Clearly, there's plenty of evidence that the threat of drug-testing isn't going to compel many of these guys to stop. But that doesn't mean the Pee Man will ever stop coming to call.