“As a European who just discovered this, I'm dumbfounded"
Actually, I am not that familiar with unions in the USA.
I have heard some of the unions are influential. I also heard that unions are not allowed in Amazon.
@Acer Unions provide collective bargaining between labor and capital.
They were very effective and why corporations have done everything they possibly can to bust the unions that effect their market.
Since the 1980’s there has been a rapid decline in union membership. Amazon is a good example of why.
If a member of a union is at-will and terminated for reasons the union disagrees with then the union can negotiate those reasons. Usually those develop into contracts which legally have no effect on at-will employment but provisioned within the contract itself.
this also doesn't account for union bargaining but you know how unions are going in the USA.