AWS doesn't make any claims to providing universal service and like most (if not all) service providers they can terminate service for a variety of reasons.
I have no inside information but it was probably a combination of some of the following factors.
1. Compliance with the AWS acceptable usage policy. AWS says that Parler didn't comply with the AUP. All the facts won't come out unless this lawsuit goes to trial and that seems unlikely. Parler's CEO doesn't seem to think moderation is important so it's easy to believe that they didn't put a lot of effort in it. It's also possible that they can't afford to comply with the AUP but that isn't Amazon's problem.
2. Cancel culture. It was only a matter of time before AWS started getting bad PR for having Parler as a client. Petitions for dropping them were already circulating internally and externally. It's easy to say that you shouldn't drop clients in response to pressure but there is a point at which doing so is a rational decision.
3. Liability. Parler was allegedly used for planning illegal activities. If another significant attack occurs then a class-action lawsuit will pursue both Parler and AWS for contributing to the attack - this happened with the September 11th attack. The costs of defending or settling this lawsuit could easily run into the millions.
Hypothetically, how would you be able to prove that point without providing examples?
Tweets from Ayatollah Khomeini talking about how Israel is a cancer[1] that needs to be violently eradicated[2] apparently don't run afoul of the "Glorification of Violence" policy. OTOH Trump's 2 tweets, which were tame in comparison, were bannable.
[1]: https://twitter.com/khamenei_ir/status/1003332853525110784
[2]: https://twitter.com/khamenei_ir/status/1263742339891298304