They banned the app because the moderation policies allowed for said insurrection attempt to occur. There are tons of posts explicitly calling for violence that violate Parler's Terms of Service, yet have not been removed from Parler, meaning that there's a failure to moderate the platform. If Parler actually moderated its content to prevent users from plotting insurrection, the app would probably not have been removed. But the whole draw of Parler is that it never would have removed that content. Since high-profile Parler users were promoting Stop the Steal and potential violence, the app's userbase would see removal as a violation of "free speech social media" and move on from Parler to the next thing that allows them to talk about it.
It's not punishing all Parler users for the actions of a small group on Parler, it's punishing Parler itself for failing to keep that small group from breaking its own rules, along with Apple's, Google's, Amazon's, etc. I don't see anything wrong with that.
