Why do we willingly allow private governments, especially those with an enormous amount of power over people? Who would ever want these companies with hundreds of millions to a billion+ users to be allowed to censor other than complying with local laws?
In my opinion, Google, Facebook, and other large corporations are a threat to our democracy & elections in a much larger way than any Russia claim that could be made. Censorship on the level that Google could and probably does pull off is a major concern and could swing elections that directly effect Americans negatively at the benefit of Google.
It's because people confuse the law with the principles it was meant to implement, so they stop their analysis after they've determined legality/illegality.
Hypothetically, you could engineer away freedom of speech and freedom of assembly without changing the Constitution at all, if one corporation owned all the land and all the communication facilities in the US. It could enact a draconian, ideological censorship regime just by exercising its property rights, and that would be Constitutionally kosher because it's not an organization classified as "government." Antitrust law, etc. is not part of the Constitution and could Constitutionally be repealed or gutted. The Constitution was written to prevent many particular forms of tyranny and violations of rights, but it doesn't protect from all of them. That's why you can't stop with it; oftentimes you have to look deeper.
Because local laws are about creating level playing fields, not quality control.
A billion-user for-profit organization is going to optimize value for their billion users, not for their re-election campaign. This is why we have the first amendment.
Should famous theaters be legally required to host all political views? Must a church host all religions?
Freedom of speech is freedom from government oppression, but you could still get kicked out of someone's house.