Can This Possibly Be Legal?

Take a look at this story reported at espn.com about the Portland Trail Blazers and Darius Miles.
The Trail Blazers declared Miles medically unfit to play and released him. If he had played two more games with the team, his $18 million salary over the next two years would count against the Trail Blazers for the salary cap and they would have to pay a luxury tax.
It turns out that if Miles plays two more games with any team, the Trail Blazers will have to pay the luxury tax. So they sent word around to the other teams saying that if they sign Miles “for the purpose of adversely impacting the Portland Trail Blazers’ salary cap and tax positions,” the Trail Blazers will sue.
Can this possibly be legal? It sounds mighty anti-competitive to me — the sort of thing that violates anti-trust laws.
Certainly they can’t keep a team from signing Miles if they think he will contribute to the team. The only possible defense Portland could have is that the team knows that Miles is not healthy, but signs him and puts him into the game just to hurt the Trail Blazers. Absent smoking-gun evidence like emails stating the motivation for signing Miles was solely to hurt the Trail Blazers, how in the world could Portland win such a case?
It might be a different story if Miles played basketball like me, but this is a player who, before a knee injury, was the #3 overall draft choice and made the N.B.A. all-rookie team in 2001. Also, if the Celtics saw fit to sign Miles in the pre-season this year, it likely had something to do with my former student Mike Zarren saying Miles was underrated. Given the Celtics’ success lately, building a team off their rejects might not be such a bad idea.