Unions are forced participation. I am not forced to participate in political parties or religious organizations. In theory a Union could be purely voluntary in which case cool. I'll opt out. But then the Union loses most of it's leverage so it in practice if a Union gets introduced it's no longer voluntary it's forced for anyone who works at that company. The only choice for someone who doesn't want to contribute to the union is to leave the company.

Government is also forced participation but I accept it as necessary part of a functioning society. I don't see Software Engineering Unions as a necessary part of a functioning society though.