All the partisan boards I've seen, left and right, all plainly labeled themselves as such. If you're going to be a place for like-minded people, that's a selling point.

It doesn't make a lot of business sense to say, "we invite everyone in" and then plan on kicking out a lot of users.

There are loads of simpler reasons why lefties thought they were getting booted, especially incompetence on Parler's part.