Pro sports players' unions.

It makes sense -- owners must act as a collective unit in order to ensure competitive balance and some degree of economic stability. A sports league where one team can afford to buy all of the best players from other teams is going to lose fans very fast. With owners acting collectively, players act collectively as a counterbalance. They make sure that they get a reasonable share of the overall income, and that it's distributed in a way that pays top performers more but also pays well enough at the bottom to keep the league an attractive place for marginal players.