Other companies have exemption policies, or mechanisms to get approvals. Amazon does not. Your choices were work privately on your own project or leave (or risk being fired for violating the policy.)
Amazon was more aggressive (in my opinion) and much stricter than any other company I've ever worked at.
Edit: I researched a lot of the other tech companies. Here's Google's documentation, for example, which outlines an exception process: https://opensource.google.com/docs/iarc/
