As it currently stands, it is not against the rules to upload paid rewards and content from banned artists. However, doing so is an inherently immoral practice and it's especially scummy when you consider that anyone can simply pay for an account upgrade to view a large gallery of posts that these artists wanted removed. In the case of paid rewards, it is very easy to see it as Danbooru essentially profiting off of artists' stolen wares.
While it is discouraged to upload this content, such warnings fall flat when there are no real consequences to doing so. As such, I move to make paid_reward posts banned by default (like banned_artist posts already are) and have banned posts only accessible to Moderators and above.
Updated by Apollyon