The workers would be creating a non compete for their own positions, and enforcing it against amazon hiring the scabs. Their right to do so is questionable, just as normal non-competes are, but we'll see how legal judgement works out I suppose.
This is all very interesting stuff. Exciting time to be alive.
I guess in your hypothetical alternative reality where Amazon had signed such an agreement, that would be a relevant question. In this one however, it is not.