you hit on the most important part. If an oppressive country's options are to block a bunch of stuff that's not all frowned upon, or allow something they really don't want, they choose the former.
I'm not saying Amazon is evil or anything, but if a bunch of hugely important customers started getting their traffic blocked by huge geographic segments due to another unrelated customer.... business is business.
And in case anyone here doubts this is true, try using Google, Facebook or Twitter in China. It is absolutely possible for countries to block these services, including AWS or parts of it. Amazon has a duty to it's other customers to protect them from that sort of impact caused by the frankly unethical action of another AWS user.
This is a real shame and I wish there was a viable option open to Signal, it's an important app that provides a valuable service, but they really should have seen this coming.
I'm not saying Amazon is evil or anything, but if a bunch of hugely important customers started getting their traffic blocked by huge geographic segments due to another unrelated customer.... business is business.