Source: https://blogs.findlaw.com/law_and_life/2014/10/can-you-be-fi...
It's similar to the way at-will employment works with ADA protected classes - sure, they can fire you, but if you can show that them firing you was because of a protected action (union organizing), you can sue and will likely win.
Given the likely fact that Amazon only cared about quarantine for this individual, only after they organized a strike, and three weeks (!) after exposure, it's pretty clear that what Amazon did here was illegal.
This is actually quite unclear