From the NBA CBA:
Section 1. No Collusion.
Subject to Section 2 below, no NBA Team, its employees or agents, will enter into any contracts, combinations or conspiracies, express or implied, with the NBA or any other NBA Team, their employees or agents: (a) to negotiate or not to negotiate with any Veteran or Rookie; (b) to submit or not to submit an Offer Sheet to any Restricted Free Agent; (c) to offer or not to offer a Player Contract to any Free Agent; (d) to exercise or not to exercise a Right of First Refusal; or (e) concerning the terms or conditions of employment offered to any Veteran or Rookie.
“The Portland Trail Blazers are aware that certain teams may be contemplating signing Darius Miles to a contract for the purpose of adversely impacting the Portland Trail Blazers Salary Cap and tax positions. Such conduct from a team would violate its fiduciary duty as an NBA joint venturer. In addition, persons or entities involved in such conduct may be individually liable to the Portland Trail Blazers for tortuously interfering with the Portland Trail Blazers’ contract rights and perspective economic opportunities.
“Please be aware that if a team engages in such conduct, the Portland Trail Blazers will take all necessary steps to safeguard its rights, including, without limitation, litigation.”
It's pretty clear to me that in essense, the Trailblazers are threatening to sue any team which signs D. Miles if they believe the team is doing it just to hurt their "tax positions." Ludicrous. The intent of any player signing should not be determined by a team who has the most to lose from it, and threatening to sue a team if they give a free agent work is definitely against the CBA.
Now for this "Fiduciary Duty" claim.
The fiduciary duty is a legal relationship of confidence or trust between two or more parties, most commonly a fiduciary or trustee and a principal or beneficiary. One party, for example a corporate trust company or the trust department of a bank, holds a fiduciary relation or acts in a fiduciary capacity to another, such as one whose funds are entrusted to it for investment. In a fiduciary relation one person justifiably reposes confidence, good faith, reliance and trust in another whose aid, advice or protection is sought in some matter.
I have not found this described anywhere in the CBA at the time of writing this post.