It's often hard to split work between countries effectively (especially in large corporations). A lot of the larger & more impactful projects tend to be done at a home base - for political or logistical reasons.
I don't know which part of Amazon you worked for - but this was totally done in Amazon: We had/have major pieces of kindle, aws, payments et. built out of India. I worked for one of those teams for two years. Amazon is so big that segmenting like this isn't a big deal at all.
I am all for shutting the door on under-qualified engineers being hired just because they will work for much lower wages, But applying the same rule to companies like Amazon / Microsoft etc would just mean work moves to those offices.
Personally I moved here because the team I wanted to work for was here (DynamoDB - given my history building storage engines before). I stayed because of the quality of life / opportunities. I mean if my visa gets revoked, I would sell my house etc here, pick up my money and probably end up in Canada / Europe offices of Snap anyway.
I try not to attach myself to material stuff too much, I'd like to think I can make the best out of wherever I am, improve when I can. But not many (including my family) are like that. They like the security provided by a good immigration policy - means this will only take high paying jobs away from here.
Speaking from experience at Amazon & Google.