Facebook, et al have moderation mechanisms that can probably withstand scrutiny, but Parler sounds like an extremely lean operation that the discovery process may reveal to have negligently failed to act on information that led to death. It's not about people disliking their political speech, it's about them ignoring things they had a responsibility to act on.
If moderation laws essentially make it impossible to build new mass platforms, then that creates a very strong case for having rights to have unrestricted open access to existing platforms, does it not? For example, the networks then wouldn't be able to issue lifetime bans at their discretion.
If your newborn community attracts so many bad actors that they become such a drag on your business that you can't operate because of the liabilities the create for you, you are a victim of these bad actors. You're not the victim of a functioning legal system.
Excuse the analogy but are you staying that if someone plans a protest in my Minecraft server, and the protest turns violent - I can be held to blame for that?
This seems a very dangerous line of reasoning.
For example, I wonder how many violent crimes alcohol ultimately led to. How many violent crimes Facebook ultimately led to. How many violent crimes encryption ultimately led to.
In a court of law, there is a distinction drawn between different events leading up to a crime, with a great deal of thought put into which of those events can be held responsible and which cannot. Now, Apple or others don't have to follow that reasoning, at least absent being involved in any court cases, but there is a great deal of wisdom in that reasoning that it would be dangerous to dismiss when politically convenient.