But yes, shit talking your employer is generally not one of those reasons.
You _cannot_ sign away these rights in an employee agreement or policy you agree to. Amazon having a "policy" against speaking out about unsafe working conditions is not a legal (or moral) defense of their behavior.
Didn't help that Google guy. The NLRB itself even issued some sort of opinion stating that he had violated corporate policy and that his firing was thus justified.
If by "that Google guy" you mean the one who got fired for posting critical memes about pay on Facebook, I'm pretty sure they were ordered reinstated and the notice was issued.
The Amazon folks fired here did nothing more than say they were concerned about working conditions on Twitter. It's hard to have a more explicitly protected activity.
Let me further clarify my statement: While shit talking your employer, you must be very careful not to step outside the bounds of what is legally protected. And with attention comes scrutiny.