Would that be legal in the US? (Intentionally deleting/promoting material as moderator without disclosing that you have been hired to do so and marking it as advertisement)

Reddit is a "private" platform, mods are free to do as the please so long as they don't breach the terms of service. In this instance I'd say the T.o.S. needs a major overhaul.

it's private in the sense of ownership but it most certainly isn't private in the sense of impact on the public. same deal with other social networks like facebook. i don't see that much difference between a publicly and privately owned psy-ops weapons which open social networks have become.

I don't disagree, we just don't really have any laws that protect free speech on the internet