I've got to disagree with you, while I disapprove of taking illegal drugs in general, I have plenty of friends who do smoke weed, and none of them are any more likely to commit other crimes than you or I. While I'm sure some do commit other crimes, that's true about anyone. You're just making a completely incorrect generalisation.
How do you know it's incorrect? You'd be making one yourself then.
I think the police realise that people who are willing to break the law by smoking cannabis are more likely to not mind breaking the law in other ways. They don't know the person, but this is a precautionary assumption, and I think it's probably often on the money. In fact, often when you're pulled for seatbelt, headlights etc, they often check license, insurance, condition of car and so on. They may be wrong, but I think it's worth checking up in the interest of reducing crime. If you're innocent, you have nothing to hide.