Basically, you can't have the pro sports equivalent of bagmen conspire together to try to come up with a way to highly compensate players in such a way as to circumvent the salary cap. The finer points of this, like how is fair market value determined so you can decide whether something is in excess of that, well that would get sussed out in court. And obviously there's some level of wiggle room, everyone knows there's more external endorsements to be had playing sports in Los Angeles than in Buffalo. But, the important part is they go mostly go after the team for this, not the individual player. Joe Smith and the Timberwolves is a good real-life example of attempted salary cap evasion and fallout. Smith got only indirectly punished because of the specifics of how the (Larry) Bird Rights work in the NBA.