The Blazers' threat is bogus. Individual teams aren't responsible for the NBA salary cap rules, and Miles has a right to earn a living in any legal way he sees fit. In fact, I'd like to see the Celtics resign Miles and waive Cassell.
If no team is willing to sign Miles, Miles might have a claim against the Blazers for restricting his economic opportunities, plus a potential suit against the league for collusion. And if he wins a collusion suit, the other teams could sue the Blazers for contribution (if permitted under the league charter and by-laws).