I'd grant that (a) platforms have a fundamental right to try to realize their vision, which may include promoting and demoting various types of content & (b) spam and astroturfing is a constant reality in any social platform (and users are better served by less of both).  

I don't think that platforms should have any such fundamental right wrt user produced content. I think that platforms should work as either publishers (where they produce and are responsible for all the content), or as common carriers (where they are forbidden by law from interfering with legal content). I think that platforms should have to explicitly choose one model once they reach a certain number of participants or when they incorporate.

I am all for shielding platforms from liability for user content if they act like a common carrier and limit themselves to removing illegal content. However I don't see why we as a society should shield companies from liability when they selectively pick and choose which user content to promote and which to suppress, according to their own preferences.