TIL that professional athletes and Hollywood actors aren't allowed to negotiate their salaries /s. Both have very high levels of union participate, yet have extreme levels of salary negotiation.

Unions don't have to involve fixed pay scales to boost salaries for the participants, e.g. SAG-AFTRA mandates a portion of revenue go to actors in addition to salaries, NFLPA requires that a certain portion of revenue be paid in player salaries, neither prevents individuals from negotiating a higher salary for themselves.