It's mostly the addict part, I think. There's an assumption that anyone who would chemically addict themselves to a harmful substance for any reason but medical necessity is, well. Stupid isn't quite the right word, but it's a peculiar sort of short-sighted that would generally preclude concern as to how badly drug X plus drug Y would fuck them up. They're already screwing themselves over, so what's a bit more...?
A different question would be, of the users you do know, do a majority of them actually have knowledge of potentially harmful interactions (especially for reasons beyond unfortunate direct experience)? If not, how large of a minority does? A statistically supported assumption is still an assumption (and able to be incorrect), but if you're looking for a reason why people assume that and most users you know fit that assumption... So far as personal interaction goes, m'self, while I've met a few (willing to acknowledge it) users that are fairly on top of things (to the extent they're able to be), most of the ones I've met are... not quite that concerned, to say the least.
All that's being charitable though, probably. It's most likely an immediate assumption that drug addict == idiot scumbag. Or something. Maybe it's only like that for the ones that get caught, I'unno.