I think there's a common-sense distinction -- obviously the USA Network doesn't need WFMU to promote its shows, and Ken needling Andy never led to either of them saying anything like "Mondays at 10! Check local listings." And WFMU DJs are allowed to have lives. During my short-lived, mostly-failed show, I was careful never to plug anything of mine, but I made no secret of the fact that I'm a playwright. DJ/Rupture would be another example -- he doesn't plug his work, but mentions panels and festivals he's worked, especially when he brings guests in. I also think that most FMU DJs don't talk much about their off-air lives because (1) it's dull radio, and (2) they want privacy.
I don't want to speak for Tom, but he has said many times in interviews that he tries to be really careful about staying on the inside of that line. I also know from listening to 7SD that Andy, or at least his on-air persona, is mostly ignorant of/indifferent to WFMU rules.