I think it allowed the government to set the stage as to labor disputes. Yes, Yellow Dog contracts were shit, but it was one way labor was fighting back hard. If a union man saw a sign saying "Now hiring, no unions!" he'd go in there and freaking sign up and cause all sorts of headaches for the employer. With the government basically saying "we won't discuss it" (by refusing injunctions) it killed the philosophical discussions. It killed the court rulings. It made labor passive with respect to employers (indeed labor cheered it, naively, imo).
Basically the Republicans of that time saw the world through a lens of the almighty contract. And Yellow Dog contracts were a contract that capitalists used to their favor (like the vast majority of non-immediate-transaction-immediate-transfer-based contracts). The Republicans hated the headache it was causing though because it showed a kind of contract that on its face broke the non-aggression principle and it had to be neutered.
What did Norris-Laguardia it get us? Well, where's the Wal-Mart union? It didn't serve its purpose in the long run because Wal-Mart can and will fire anyone who wants to start a union, and it's not in the contract at all! And, because the government won't form injunctions (this is in the event of a mass firing and employees suing Wal-Mart to allow them to keep their jobs), it's not discussed! It's a double edged sword.
What FDR should've realized is that labor disputes should be covered by the government, and not in some sort of set way, but rather, the government should've said "We will look at every labor dispute in a case by case basis." So, when factory workers took over a large baron's shipping company, and they did so wholesale, the discussion about whose property the factory really is would take place.\
Note: Norris-Laguardia did, importantly, say that forming unions did not equate "conspiracy," but I think that part is just common sense really (since unions are merely ones expression of free speech and association). Still, that would've been part of it I think was good.
By NLRB I meant the Wagner Act, my apologies. FDR signed it into law. This created a hierarchy within unions, limiting the power of autonomous union actions. Anyone could form a union, but they needed to select a leader, which went against the original concept of free association and autonomy. This is the "set way" I was talking about. Because all labor disputes are the same, they never actually result in much direct action or strikes or appropriation of capitalist property. It's clean. Board room dealing. And the working class is ignorant of the whole thing because they don't generally experience what labor disputes were like back in the day. Taft-Hartley was an amendment to the Wagner Act and it and other legislation ultimately legitimized stealth yellow dog contracts.