They have a right to form a union. Amazon has the right to fire them at will. Unions create an antagonistic relationship between member employees and the company that employs them.
For example: https://www.msn.com/en-us/news/technology/us-labor-board-say....
And: https://www.nlrb.gov/about-nlrb/rights-we-protect/your-right...
The insane and rabid anti-union belligerence in US corporations is just as responsible for any antagonism. Other countries don’t have anything like the insane behaviour, because there isn’t one side willing to go to literally any lengths including shutting down sites to avoid working with unions.
A much better way to avoid unions is to not abuse your employees, that way they don’t feel that they’re being mistreated due to a power imbalance that might be fixed by a union.