The NBA has a revenue sharing program. There are 29 other NBA teams. Because of the stupid comments and subsequent boycotts, it is devaluing the investments and costing the 29 other owners real money.
And you can bet his franchise agreement provides the ability of the NBA to monetarily penalize him.
The NBA has every right to protect their business interests.
Only for specified acts and whether a private conversation constitutes "detrimental conduct" will make for a very interesting lawsuit. Where would that stop? If an owner state in a private conversation that he disagrees with gay marriage can the homo crowd exercise a heckler's veto and strip the owner of his franchise by instituting a boycott?