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).