Sports unions are a bit of a quandary. In part I think it has to do with the widespread differences in talent and therefore earnings potential within the union membership.
That said, one thing I always found odd was the acceptance of one-and-done college players in the NBA. Unions increase membership bargaining power by restricting entry, and therefore should keep college players in college as long as possible. That only makes sense for the benefit of current members.
The more players added to the ranks the less bargaining power those current players have.