Presumably a maximum pay is set with the intention of allowing that money to go elsewhere in the sport? The NBA knows stars will get paid very well, but they want to ensure not all the money is spent on them so that lesser known players are fairly compensated as well. It seems a smart idea especially because an instituted max is still allowing stars to be paid 8 figures a year.
IMO, the maximum pay/salary caps are set to promote competition across the league. As an entertainment product provided by a monopoly supplier, that's exactly the right thing to do. As a (clearly hypothetical if you'd ever met me) top talented employee under such a scheme, it's terribly counter-productive to my individual situation.