It's not necessarily either. It may very well simply be the preponderance of the evidence. Nevertheless, such a suit will be undertaken with the benefit of the discovery process.
But if the plaintiff produces no evidence, Amazon does not need to make a defense. Thus OP is correct.
Even discounting all of that, the judge/jury/arbitrator/litigator would have to agree that sending him into quarantine and not others constitutes retaliation. To be completely honest, this kind of job is a huge joke. If you take too many bathroom breaks you won't hit your quota and they cN fire you for that.
The only way to win isn't to prove he was treated inconsistently, that can be ignored so long as the reason they stated for letting him go is true.
And arbitrators are always required to be agreed on by both parties.
Sure, but any evidence which makes an accusation more likely than in the absence of that evidence suffices to meet preponderance of the evidence in the absence of any contrary evidence. The fact of the labor organizing, the fact of the firing, and their temporal relationship are, together, evidence for retaliation.
So, Amazon will very likely need to make the case (and Amazon will need to present the evidence to support it), that he was actually fired for violating the company mandated quarantine.
The actual evidentiary fight will probably be over whether that quarantine was a bona fide quarantine, or a pretextual one. But who has the burden to present that evidence will very much depend on who feels like they're losing the case. Probably both of them will need to present evidence to support their position.
This sentence is simply false.
You have a reasonable indication, but no preponderance of evidence. You probably have enough for discovery.