Most of it actually wasn't FWIW, hateful extremist content is generally perfectly legal free speech. "Incitement" gets used way, way too often on the internet, almost nothing that gets posted online is legal incitement. But neither "Big Tech" (such a dumb term) nor Hacker News nor a random forum on birds needs any violation of law or anything else to moderate what gets posted on their sites. It doesn't have to be "negative" or whatever at all even. There is nothing illegal or objectionable about someone who likes discussing trains for example. But if you post lots just about trains on a birder forum they may delete all your posts and ask you to stop because they want to focus on birds, and if you continue to do so they can delete everything and ban you. Why would there be anything wrong with that?
Private society looking at extremist content and saying "we're not going to shoot you over it but we do strongly object and we're going to socially ostracize you and deny you business and our support in any way we can" is free speech working as intended.
>Is "seems like" enough of a reason now for private companies to choose not to contract with other private companies?
Uh, yeah? People can refuse to do business with each other for nearly any reason at all, and definitely for anything other people merely say or do (at least, within the bounds defined by any existing contracts, but Amazon has covered its bases pretty well there to put it mildly).