It really is something that needs to be delineated, but I would assume (perhaps wrongly) that premium series creators have been required to sign some contract with more specific terms. If so, there is absolutely no reason, nor is there an excuse, for attempting to force non-premium users, who may not receive any benefit whatsoever other than free hosting, into the same terms. Not everyone opts into the ad revenue program, and those who do are provided with a meager compensation for that, as has been adequately noted; and that isn't even the real program intended for commercial benefit.
This is a serious problem. Anyone with industry experience will likely recall Tokyopop and its problems with similar practices. By the end, no creator would contract with them for similar concerns, and for good reason: even to this day, intellectual properties of creators are still tied up in their problematic contractual entanglements.
After Tapastic altered their name and removed the forums -- the main and only direct community nexus for interaction -- to hide them away at the bottom of a secondary page, I find it very hard to believe that community engagement is a priority. The forums have never been a priority, and the persistent issues with it prove that handily -- most new users can't even upload an icon to represent them. As such, I find it very difficult to believe that any real consideration has been given, or will be given, to that community that is responsible for the site being built up to the level it enjoys at present. I have seen too many sites do the same and then end up disregarding everything the community says or asks.
If these terms persist for all users, I will have no choice but to attach a proviso to all my comics that remain until I choose to walk away, which will be at the end of the current chapter -- quite soon -- for my single currently running comic. I have accepted Tapastic's limitations up to now, because they were a free host that seemed to have some degree of mindfulness about promotion and reaching a broader audience. But frankly, their terms are too restrictive as it is, even without consideration of the possibility that they might be encroaching upon my legal rights of copyright and commercial ownership.
I literally have to be able to make the call on whatever I need for my series. If you have not entered into an explicit and exclusive commercial relationship with me, through an explicit and exclusive contract with me, you do not get to claim any such privilege.