"Other people are using their own free will and self determination to undermine a CORPORATION!"
Cartel behavior requires coordination and collusion. If they are just following general social/business practices without having meetings about who to blacklist, then it doesn't meet most definitions.
Historically solidarity between workers is the reason why you only work 8 (instead of 16) hour days. Why workers have vacation days, sick pay, pensions and maternity leave etc.
Why? We don’t trust the state to do it better. Neither employees nor employers do.
The TL;DR of US labor law is:
1. Companies do [very illegal under local law, up to and including murder] things to bust unions; they buy off the local authorities to get away with this.
2. The Federal government adds specific protections for unions -- many of which don't exist in various European countries.
3. Companies complain that #2 makes unions too powerful, so the Federal government adds specific restrictions on unions -- many of which don't exist in various European countries.
I'm curious about the history in Europe, since an equilibrium seems to have been found without steps #2 and #3.
Also many unions did what business owners had done for centuries and went into politics. The social Democratic party in Sweden for example, is a part of the largest labour union. This is somewhat controversial in some circles, for sure, but changes the power dynamic for steps 2 and 3 a lot.