He violated the terms of his probation, that's all there really is to it. It has nothing to do with the legality of what he partook in. If his probation included a no alcohol clause (which for many offenses they often do) he would have been in violation if he was caught drinking. And based on what has been presented in the news, it doesn't sound like he was arrested for weed, he failed a drug test which put him in violation. He is not serving any new jail time; they just are making him serve his delayed 30 day for the original offense immediately.