They have a TOS and policies like any other company. You can get kicked off AWS for much less than hosting a service that willfully turns a blind eye to domestic terrorism.
What existing law do you think prevents Amazon from removing Parler?
Or how about this.
Would you like the CEO of Parler to be telling you what you are allowed to write?
That is the consequence of what you are saying.
Parler moderating Parler; Twitter moderating Twitter; are very different from AWS moderating Parler or Twitter.
Parler is violating the terms of service, AWS is asking for minimal level of moderation to avoid violating those terms, and they are refusing.
And the CEO of parler does tell people what they are allowed to write on Parler they remove so-called "leftist" content all the time.
If im running a website or service, why shouldnt I be allowed to remove content from it? Wouldnt a law thats preventing me from removing that content actually be a violation of my right to free speech?