And if the solution was to force Amazon to block any access from their servers to those oppressive regimes, that wouldn't help Signal at all, because they too would be blocked.
For the record, U.S. politicians have voted on sanctions on Russia for cyber crimes and brought representatives to the UN raising the issue of their human rights records.
Indeed, the US nearly destroyed Russia's largest aluminum company - Rusal - recently in a sanctions move against an oligarch close to Putin (Oleg Deripaska, who owns the majority of Rusal).
The best way to deal with oppressive regimes, is generally to go through powerful political bodies/groups, whether the UN, G7, or US Congress. The impact a company like Amazon (or Google, Facebook, etc) can make is very trivial. So trivial as to be meaningless to a typical oppressive regime. Congress, in tandem with large allies, can hammer eg Russia's primitive industrial economy, by comparison, with targeted sanctions on steel, aluminum, whatever.
"On April 23, however, the US government gave Rusal's American customers "more time to comply with sanctions", even saying it would "consider lifting them if United Company Rusal Plc’s major shareholder, Russian tycoon Oleg Deripaska, ceded control of the company." Department of the Treasury gave these clients until October 23, 2018 to comply with (wind down business) the Rusal sanctions."
https://www.reuters.com/article/us-usa-russia-sanctions-rusa...
For the record, Deripaska is no friend of Putin, who forced him to start paying tax and stay out of politics. Look at how Putin humiliated him several years ago during an industrial dispute, when Putin took the side of workers against Deripaska.
Look to Mikhail Khodorkovsky to see what happens to those that aren't Putin's friends.
If you want Amazon to stop doing business with oppressive regimes, contact your politicians about sanctions.