Are we just assuming this userbase doesn't also have twitter, facebook, signal, etc accounts? I recall a lot of live streaming and tweets during the riots.

Exactly. I already had that factored into my prior for whether my business was going to get cancelled by Amazon. I’d actually expect to fade a court for knowingly facilitating the shit on Parler.

And if my hypothetical platform is decentralized to the extent that I can’t top-down censor content, what then?

Then you probably shouldn't host it in AWS, because it'll risk violating other AUP terms.

The Acceptable Use Policy is here: https://aws.amazon.com/aup/

If your users use your service for illegal or fraudulent activities, you have no way of stopping them, and your service is on AWS, then you risk being shut-down.

That hasn't changed today. That was true as of September 16th, 2016 (and probably before then as well)

That is simply a different question