I feel like you are playing semantics here. "Forced to join a union" in this discussion is in the context of "in order to get a job at Company X". Of course this worker is free to forego employment at Company X if they don't like the terms of the employment. However, one of those terms forces the worker to join a particular union.

Taken to its logical conclusion, this semantic distinction could be used to justify eliminating just about any worker protection currently on the books.