These companies didn't wake up yesterday morning and decide Parler was "bad". There's been a power shift in Washington, and they are trying to signal to the new people in power "We're on your side, we'll listen! No need to regulate us!"
That's the real problem here - there's more to this than a private company deciding who is on their platform, which is their prerogative. There is implicit (and explicit) political pressure to do so. That's what is scary and, in my opinion, wrong. And it isn't a left-right thing. I'm sure the Republicans are looking at this and getting the same message, and would be just as happy to use the same tools.