When the workers finally unionise at every warehouse, Amazon can rest comfortably in the knowledge that they caused it.
Firing an employee becomes much harder. You have to convince the union first.
Employees can file grievances. If a disciplinary or surveillance system is considered onerous (by them), then they file a grievance, and much bureaucracy ensues.
Pay scales tend to depend on seniority and job classification, not on a supervisor's idea of your merit. This is negotiated in the union contract every few years, as a rule.
Major changes in how the warehouse operates have to be cleared with the union.
Union dues are deducted from paychecks.
The union can go on strike, and other unions generally respect their picket lines.
I'm deliberately not making this seem like Heaven on Earth. That's probably why union elections are contentious.
With a really shitty employer like Amazon (at least at some of their warehouses), a union really might provide some protection to the workers. All those stories about workers having to pee in a bottle because they get docked for too many bathroom breaks: gone. The union would take care of it.
Getting fired for too many demerits from the overhead surveillance cameras? Gone.
Of course, when you vote Yes or No for a union, you don't really know if they'll be able to negotiate all that.
Prior to the 1930s, unions provided some on-the-job safety, and some protections against arbitrary management or pay cuts. Over time, those got written into laws or companies just refrained from really shitty practices. Most companies, anyway. Not all.