Private companies are free to ‘censor’ what they believe is either bad for business or exposes them to liability. Maybe the free market has decided extremism is bad for business.
So did the press, and yet they were expressly granted this freedom. Buying a printing press and starting your own paper wasn't easy either, but the 1st amendment never said it was supposed to be.
I'm not concerned about social media network effects. MySpace and AOL used to be dominant services.
Tangentially, not terminating business with Parler could be construed as not operating in their shareholders best interest, a breach of their fiduciary duty, as it could potentially harm future business opportunities if they become associated with the platform. In this case AWS is not a utility that holds a monopoly on some resource. There is market competition, and hosting your own service is an option. If a telcom prevented the latter I'd agree with your argument, as there aren't necessarily alternate options. Even then the law outlines a framework where some content is illegal that any lawyer working for the firm could green flag for client/contract termination.
You are not forced to use those, nor any social media platform. Nor are you entitled to the use of any social media platform, legally anyway.
You WANT to use them, but that's much different from being forced. Being forced would mean everyone who gets a US drivers license also gets a facebook account.
https://en.m.wikipedia.org/wiki/Outcast_(person)
https://en.m.wikipedia.org/wiki/Excommunication
Acting/speaking on specific ideas have caused people to be outcast, “social pariahs”, and excommunicated for millennia.
What is new today is the scale that people can reach, and the ability to excommunicate a person’s scale without excommunicating them.