I think the right mental model to have here is that AWS can boot any customer that becomes problematic for them, including Signal. It seems unlikely that you can skirt their AUP terms by constructing a service that makes it impossible for you to comply with their terms.

Amazon can enforce their contracts selectively. The contract isn't a statute; it's an agreement between two parties. Amazon could presumably strike up a side agreement with a customer that overrode the "master" contract. I'm not a lawyer, but stuff like that happens in consulting somewhat regularly, where you have an MSA governing all your projects, and specific contracts for weird projects.