because his fans are crazy to like him in the first place. Sorry, but it's true. He sounds like a girl, looks like a girl, and does/did drugs. I still think I like 1D, but they don't do drugs. Zayn might smoke, but I do applaud him, because he is trying so hard to quit. the others, well, they might say their idiots, but they at least know how to look totally adorable while being idiotic too.
Because they are very big fans of him. I guess it's their way of trying to get him to stop, however it won't work. People need to be smarter about it. If you did a drug that you enjoyed and people started cutting, would that make you stop? I see it as fans over exaggerating. I myself dislike justin bieber and could care less of what he does. the only reason I know about anything are from my friends who happen to be fans. luckily they're smart and are among the few fans who didnt cut for bieber.
Who knows why. Their priorities in life are not in order at this point. It was only marijuana. it's not like he's snorting coke or shooting meth. People of any age should never let somebody else, famous or not have that great of an effect on their lives.