Some people signed the contract thinking it's a decent place to work but found out later it's not. They received relocation bonus (which they have to return if they leave within 2 years), left their previous job, are now in a country where there visa depends on being employed by Amazon so things are not as black and white as you would like to represent them and they just can't leave as easily as some. It's not slavery, but it's not far from indentured servitude.

P.S. I'm not in that situation but have a few colleagues who are, and going back to Ukraine for them is not an option.