They are? You just pay $100 (or whatever) and it gets posted. The only curation that might happen is the fact that a human probably has to manually insert it into the draft, because the newspaper hasn't developed proper automation for this sort of stuff. Moreover this is easily side-stepped by replacing "newspaper classifieds" with "ads stapled onto power poles" or "ads placed online" (in which case it probably is automated and there's no human review).