I don't think it is saying "oh they can't because they have a contract that says they can't", especially based off the context of the rest of the article. I think it is basically saying unions don't really have class conscience or militancy and are more worried about getting a contract even if that includes "signing away" your right to strike and have generally been tamed. Then it is saying that unions need their own "mini revolutions" where they wouldn't even rely on contracts(and have the capacity and organization to fight without relying on and being tamed by the NLRB) but that is likely a long term project. Basically, being critical of how many unions work and them relying on contracts but breaking this "habit" won't happen overnight.