The NBA has what’s called a “soft cap,” which means that teams can exceed it if they really, really want to, and most teams really, really want to.

Here’s how well it works in practice: of 30 NBA teams, a whopping three are under the cap — Memphis, Charlotte and Atlanta. Of the remaining 27 teams, 18 are at least $10 million over the cap. The top payroll when the season began belonged to the Mavericks — $104.5 million, a mere 90 percent over the cap, which in owner Mark Cuban’s world passes for a pittance. The Nets, at $67.7 million, are $12 million over the cap.

Why even bother with a cap? Since teams spend whatever they want anyway, why not just do what baseball does and let teams spend to their hearts content — but charge them for the luxury of having so much money to throw away. Okay, so under that system, Van Horn wouldn’t get paid millions to spend the month in New Jersey, but even he says he doesn’t really need the money. He’s just taking it because it’s there.

If I understand the cap correctly — and I’m positive I don’t — the Nets have to take Van Horn so they can keep spending more money than they need to on a team that plays with little enthusiasm in a half-empty arena every night. Either that, or they have to take him so the Mavs can continue to pay a kings’ ransom for a team that will probably be eliminated in the second round of the playoffs.

But the why of it doesn’t matter. The fact is that the NBA’s rules demand that a guy who doesn’t want to play basketball spend the next month in New Jersey pretending to practice so that an incomprehensible rule that no one follows will remain inviolate.

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You know the rule is incomprehensible because none of the stories that the Associated Press has filed on the trade even bothers to fully explain it. The stories just say the salary cap rules made it necessary.

If you’ve got so little going on in your life that you actually do understand it — I know, it has something to do with “matching salaries” — please don’t bother trying to explain it to me. Life’s too short to attempt to understand something that has so little meaning.

It’s a joke, and if this is what it takes to satisfy the beancounters and labor lawyers, the NBA is better off without it. If it doesn’t work, don’t fix it. Throw it away.