Because he is in breach of contract. Pro sports Leagues have rules/obligations you must follow to be permitted to own a team. Essentially, it’s a club with a contract. They can get rid of you in many ways for “conduct detrimental”.
Free Speech etc does not apply as this is not a government action.
I can understand the contract issue but do not see how that falls into the conduct arena. But I suppose it all comes down to what is stipulated in that contract.