Well Amazon has a history of mistreating workers. So, why do we think they are likely mistreating workers? It's because they have a long & well-document track record of doing so.

https://www.theverge.com/2018/4/16/17243026/amazon-warehouse...

https://www.theatlantic.com/technology/archive/2019/11/amazo...

https://time.com/5629233/amazon-warehouse-employee-treatment...

And the wikipedia page: https://en.wikipedia.org/wiki/Criticism_of_Amazon