That's a good point, and something I didn't initially consider, but I think this should probably be monitored very closely otherwise we could see some worthless altcoin tokens being offered, and the forum starts to get plastered in sponsored threads. This is unlikely as it would probably be regulated before that would happen, but I think going the route of asking permission before doing it is probably the better idea.
It is possible to require Pre-clearance to have a sponsorship thread, however there still needs to be clear guidelines as to what is allowed and what is not allowed, or else there will be controversy when certain requests are denied that are similar to threads that are allowed.