The law is pretty consistent about the idea that any paid endorsement has to be "truthful". And as we've learned more about sociology and advertising, we've realized that things like paid endorsements are fundamentally not truthful, because they are misleading the public to believe that some figure or trusted source (even if only at a subconscious level - which is still enough to change consumer behavior!) is in favor of a product or brand.
So maybe if you could argue that paid speech is inherently untruthful (which I believe that it is!), then you could make legal policy that bans paid speech complaint with the First Amendment! (caveat: I am not a lawyer, I am not a legal activist, etc)