Dude, did you just get done watching the first half of Demolition Man, then?
People want to be able to do things that are bad for them - living in a society where you do things that are only good for you is fucking pointless and stupid. So yeah, even if people "believe" (read, in most of those case: know) that something is bad for them, they're going to do it. That's why my ass is fat, why my boyfriend smokes, why we both drive too fast (or drive at all - we're better drivers than most other people on the road, so they're probably more dangerous to us), why we clamber into places we shouldn't on the sides of mountains, etc.
People want to be able to do things that are bad for them - living in a society where you do things that are only good for you is fucking pointless and stupid. So yeah, even if people "believe" (read, in most of those case: know) that something is bad for them, they're going to do it. That's why my ass is fat, why my boyfriend smokes, why we both drive too fast (or drive at all - we're better drivers than most other people on the road, so they're probably more dangerous to us), why we clamber into places we shouldn't on the sides of mountains, etc.