"I saw them advertise new teaching jobs on Dave's ESL Cafe site. I am just wondering why Dave lets them advertise there..."

Coz Dave gets paid by them for the advertising, simple as that! And if anyone dares to give bad publicity to any of his advertisers by posting something negative about an advertiser, he (or she) will be asked to doctor the post. Failure to do so will result in a ban! And then Dave will remove the 'offending' post!
Contrary to what another poster has said re someone having been banned by Dave, it's not necessarily something to be ashamed of to have been banned.