We are in absolute agreement. Frankly, I have argued before that they are already too big.
"There's also a big difference between criticizing the company's products and criticizing their business practices."
I am not sure I agree with that statement. Would you feel the same if the employee publicly criticized a company for using non recycled paper AND they have to change course now and use recycled paper; afterwards the story "went viral" and employee got fired for that specific action. Is employee justified in doing that without any repercussions? It is genuinely hard for me to argue for the employee here.
"Where "the line" is depends on the specific circumstances, but it's pretty clear to me that Amazon overdid it in this case."
To you it may be clear, but clearly not to everyone since we are having this discussion. Whether the pendulum should swing ( it should ) is a worthwhile conversation to have.
"If they found these employees' statements embarrassing enough to fire them, then it sounds like they know they are doing something wrong."
I don't know if I buy this argument. HR does not like troublemakers seems like more plausible explanation.