Apple told Parler to moderate their extremist content, and Parler refused. At that point, if Apple left Parler on the App Store, Apple would be complicit. Same story played out for all the services.
And guess what, treason by definition is infectious. Giving aid to an enemy of the United States. So Apple at that point would be opening themselves to a huge legal liability if they kept the app available. Nothing has been proven in court but big tech is naturally risk adverse.
If Parler has agreed to moderate extreme content, even if they had done so dragging their feet, they would still be alive.