Dude, Tommy smokes pot and talks about smoking pot in every party he goes, IN THE OPEN, he only don't smoke around campus because it's against the law. He doesn't care if other thinks he is a scumbag and all negative stuff possible. He doesn't care that the dormies tried to say that he had STDs, he don't care about his dick being all around the campus.
He is pretty similar to Lily and Quinn in this regards, he does not care about others seeing him negatively. He would actually own it and dismiss it. And most importantly, it's that he is trying to hide it from the DIKs themselves, why would he try to hide it from the DIKs? They are the ones that know the most about how much of a crackhead Tommy is, why would he care?