What do you mean they HAVE to spend a certain amount? The BRI dictates what they can spend and can't spend. The fact is they're spending beyond their means. Everything outside of that 57% is theirs to decide how to spend, that's the part that matters.
No offense, but this is why I said you don't understand the system. The players get 57% of BRI guaranteed. Let's say for simplicity revenue is $100m for the year and player salaries are $50m. That is $7m short of the mandated 57%. So what that means is the owners have to pay this amount to the players at the end of the year. No matter if revenue is $100m, $250m, or $1b owners HAVE to pay 57% to the players.
Now let's say you're an owner and GM, and you realize there is a salary shortfall. You can either over pay to sign a guy to an extension or to sign a free agent. Or you can do nothing to try and improve your team, but it's going to cost you in either case. That's why teams "over spend"; they actually aren't they're just spending their mandated 57%.
Are you sure that's right? That makes absolutely no sense to me. In that scenario if a team had, say, a basketball related imcome of $200 million, then they would have to pay their players $114 million of that. Then, on top of that, they have to pay the luxury tax for being over the salary cap.
If that's how it works then it really is a foolish system. I mean, if the salary cap is in place to level the playing field and create parity, I don't see how this system helps to accomplish that at all. It basically forces the richer teams to go over the cap so they can pay the players.
I think it's more likely that the salary cap is based on 57% of the overall BRI of the league. Ergo, each team's individual BRI doesn't have an impact on that particular team's salary for a given season.