Is that guaranteed? Parler has found that the machines their code ran on could be taken away for having bad opinions. I'm not a defender of fascists, if anything I lean towards being happy when their megaphone is taken away, even when they receive the occasional punch, but the precedent this has set with the power of these companies is frightening for anyone.

What if Amazon will refuse to do with business with people who support unions next? It no longer seems impossible to ke.

Amazon contracts haven't changed, they always have had the right to drop you for any reason.

Amazon is not a common carrier: the cloud, other people's data centers, have no legal requirement to be neutral about what runs on them (and it's not even clear how this would work given the difference between compute and communications).

First of all, there is a difference between current law (which means that what AWS did with Parler is indeed perfectly legal) and a theoretical "perfectly fair" law, which is essentially what people mean when they say such and such should/should not be legal.

Secondly, there are limits even in current law on what grounds a business can refuse to do business. For example, Amazon could not refuse to do business with someone because of their skin color, or because of their religion. Extending this in more of a free speech direction (where companies could be prevented from refusing to do business with organizations whose message they do not condone) is not unthinkable (though there are pros and cons).