Worker rights are essential to this issue. That's why Dershowitz's argument is flawed. 

A collective bargaining agreement limits what an employer can do, and in the case of the NFL contract with the players it certainly does. It is subject to negotiation between the parties, which are co-equal under the agreement. Each side is equally charged with upholding it; each side has its own rights and responsibilities.

If there were no CBA, no union, then employment would be "at will," and the employer is relatively free to make its own rules and discipline as it sees fit. Seems to me that's what Dershowitz was relying on, and that's why he's wrong. This is not an "at will" employment relationship. 

As I pointed out before, even nonunion (aka "at will") workers have rights to "concerted activity" in their own defense, per the NLRA. It's all in my prior comment.