@Meme-Bratt
Well, as I understand it a strike is a refusal or withdrawn of work while under contract.... when the contract ends & no new one is in place, that is not a withdrawal of labour its the end of the contract..... indeed, the employers have been trying to get a negotiated extension to the contract while talks continue, but the unions refused... hence, no contract & so no legal agreement to work at all.
Looking back at other strikes, $F $CSX just two that pop in my head, the legal right to strike isn’t tied to the contract. The national labor relations act ensures that. Workers are expected to work during expired contracts being negotiated and almost always do.