Some look like they're breeders selling off the cheaper ones... but more of them come with a tank and heat lamp and presumably came from a pet store of some sort. Not all are normals necessarily, although plenty are. I doubt the humane society cares about that distinction, though.
Incidentally, some of the ads that look like they are breeders selling off the cheap ones don't inspire a whole ton of confidence either. It is not at all hard to imagine someone picking up a snake from one of them, and then a used tank and lamp from another CL seller on the way home... and that only reinforces the oversupply argument.
I don't feel qualified to make value judgements about anyone's breeding operations, and I do appreciate that it's easy to get a captive-bred snake instead of a wild-caught imported one. I'm just saying that if the humane society's goal is to reduce the number of unwanted pets out there that suffer or have to be euthanized, asking people not to make more babies isn't an unreasonable request.