I take your point about legality. But, I'd make the point that we don't have a clear moral or legal concept of free speech that relates to the current world.
"We can argue about whether or not Google should ban certain opinions on their platform, or where the line should be drawn, but it is arguable that hey have the legal right to do so."
At some point, the distinction between legal and moral breaks down.
In theory, google could legally do quite a lot. Say google decides to censor all mentions of the tiananmen square rising. They could remove it from search, youtube, android phones, chrome. Gmail could spam filter emails mentioning it. They could exert influence outside of companies/services that they own directly. None of that is illegal (at least not unconstitutional). In practice, this is very close to what China does with the great firewall.
It also wouldn't stand. Something like this would be too contradictory to the moral concept of free speech.
Constitutions are interpreted, and the supreme court is not the only interpreter. It's a cultural construct as well as a legal one.
We are effectively at this point now. Google, Twitter, Facebook, etc... These aren't platforms in the way newspapers were. They're not platforms at all. They're the level ground, in terms of speech, press, the right to petition the government or practice religion. The reasons that amendment was written runs through google.
