If it was actually on-site, yes, Amazon did the right thing. If it was by the street--somewhere any member of the public has free speech--no, but assuming he was exposed to SARS-CoV-2, going out in public is pretty messed up, and pretty hypocritical.
>>dehrma+(OP)
It's generally legal to fire employees for "free speech" activities done outside the workplace.
More morally hazardous of course, but I don't think anyone would really even challenge the cause here (it's trivially easy to argue that this behavior shows they would endanger workers within the workplace as well).