I think Gab and Parler should face no legal sanction for hosting content that is legal but awful. But people critical of that also have freedom of speech, and everybody involved has freedom of association. Telling Gab's vendors that maybe they should stop helping Nazis is just as much "in accordance with US laws" as anything on Gab, so I don't see on what grounds you could object to it.
So Gab and Parler should be absolutely free to operate as they wish, in accordance with US laws?
(Because there many folks who like Twitter's moderation who also believe Gab and Parler should be run off the internet, and have tried their best to do so)