Don't make this about 'laws' - this was definitely not about 'laws' or the 'law' would have stepped in - and it's definitely not Amazon's job to enforce the law.
Amazon knowingly sells fake goods and suppresses unions so let's not give them the moral high ground.
Apple's concern I don't think was arbitrary: there was a lot of 'really bad discourse' on Parler, and it was not moderated at all (or community moderated which had no effect).
I don't like Apple's App Store 'monopoly' however they were within reason to not want Parler in the store.
If Apple allowed non-AppStore downloads, then I think it would have sat much better.
Amazon however, I think did something a few steps more inappropriate and more nakedly political.
Amazon has a zillion customers that do arguably more nefarious things, in the 'big picture' they look hypocritical, though they are surely within legal rights as well.
I do feel however, that Amazon shouldn't be allowed to drop customers without a 3 month notice unless said customers have done something actually illegal, and by that I mean 'illegal as designated by an actual Judge'.