It's possible that he was just overly cautious and that there may be some legal grey areas. My guess would be that staff can be seen as official representatives of the forum, so anything endorsed by them could be seen as officially supported by the forum (and its owner).
I've done some digging and looks like Theymos changed his approach to advertising on the forum altogether back in Sep 2022:
https://bitcointalksearch.org/topic/m.60976136
He effectively ended the auction-style ad buying, quoting difficulty to determine which services are legit as the reason.
So maybe this has something to do with it.