[harryh]

The WNBA players union negotiates a contract with the league that includes provisions like these. In general wage caps are included because the union wants to "spread out" money away from the top stars to raise the salaries of average players.

These sorts of arrangements are common in many industries with a union represented workforce.

It's worth noting that the WNBA Union contract includes a provision for 50% of revenues to go to player salaries. This is the same standard in the men's league. WNBA salaries being substantially lower than NBA salaries are a function of league revenue being far lower.