I've been a dev at Amazon. They don't hire devs as contractors. They hire the people that support the devs. Testers, validators, vetters, etc. If you need someone to manually poke, prod, click on a button all day long, they hire temps for that. The testers for our app were temps, same thing with the people that would click the "approve" button on user submitted images or uploads.
This is not correct. I left Amazon last year after 3 years. I had a temp sit besides me for about 1 year. He did the same job as a SDE-2 would do. He worked for a team writing software for inventory management, my team worked in the same space. A month after he left, my manager asked me if I knew anyone who would be a good fit. I immediately mentioned his name based on what I had seen for over an year. I never heard back again.
Amazon has less then stellar hiring practices even for full time employees. From personal experience in no particular order (a) low balling new hires in salary and position (b) bar raiser ego trips and (c) if it isn't my answer it isn't the right answer.
I have also seen an engineer promoted to Principal for having accomplished nothing during the entire year (imo), and another principal say to me, why would anyone who has an offer from Google ever pick Amazon.