I'm not saying they're idiots, and I'm not saying companies should fire employees for airing grievances... but publicly reporting on bad things your employer is doing isn't a good idea if you want to keep your job. I'm not sure why that's surprising to anyone, including the people who got fired.
Clearly Amazon is being a bad actor here overall, both in how they've been not caring about their warehouse workers' health, and how they fired these employees for making a stink about it. But, in general, I don't blame a company for firing employees who trash talk them in public. It's just... kinda common sense that this would happen? But in the specific case of a company doing something like Amazon is doing now, I really want there to be legal protections, because employees should feel safe from retaliation if there's something whistleblower-worthy going on.