Absolutely right, nothing illegal about it. If he throws in a test sample of pot, then it's illegal. May not be a smart ad to run, especially if he is targeting ANY other market than pot buyers, but if that is his target market, this ad will do it. In any case, it gets attention (like this thread), so maybe it isn't stupid, either.
If I were the police, I wouldn't go after him. I'd start invisibly investigating who is buying from this guy, and investigate them... leads, leads, leads that could lead to convictions.