Cult TV show actress, indie video game developers and cult comic and adventure show host who are memes on Reddit. Reddit fancies these people, and the fact that they didn't pay to post makes it seem more legit. While they may be marketing something, the fact that it's not in your face makes it seem more innocent. Having something on there that specifically states "this is a sponsored AMA, someone is doing this to sell you something" immediately creates a negative reaction. Hiding the sponsorship would just cause paranoia.
Would sponsored AMAs be front paged immediately? Could they be downvoted to hell? Would comments that the sponsor found offensive be removed? Would a company want someone linking to r/SpaceDicks in their AMA? It's kind of a Pandora's box promoting on Reddit.