Unions like to set salaries, because there are more mediocre performers who stand to gain compared to free-market wages than top performers who stand to lose.

Unions like to set salaries, because there are more mediocre performers who stand to gain compared to free-market wages than top performers who stand to lose. They don't exactly want a cap, but they greatly desire a floor, and accept it also serving as a cap as an acceptable tradeoff. Whenever that first part isn't true for a union, then the market pressure is for the top performers to leave the union for free-market wages, until it is.