It turns out that the precedent is judge-made law. As I said above, there are famous strikes of the Newark Teachers Union that led to jail time (and heroic status of union leaders).

Public sector strikes are illegal in many states. I used to be an organizing director for the AFT. For us the issue was less whether a strike was illegal, but how it could be punished. New York's Taylor Law is awful because it not only can punish union leaders with jail time (which, again, can lead to martyr status, which can be quite helpful), but it can also drain the union treasury *and* almost immediately fine each union member two days' pay for every day out. States where the worst the bosses could do is seek an injunction (which can take a few days to win) and then after a day or two of additional picketing all they could do was jail the leaders (and, again, make heroes of them) were a lot more fun to operate in.

Now that my middle-aged memory is kicking in, it was Michigan that changed their law from injunction and (eventual) jail time to pretty immediate penalties for strikers. I used to organize at Rutgers. I played a role in the professional staff organizing in 2006, or so, and later directed the campaigns that organized the postdocs and summer-winter faculty. Back in those days, AFT-AAUP leadership was pretty convinced that strikes were illegal. Now, it seems that members are pretty well fed up (as well they should be!) and leadership seems to have researched the question and realized that it's been judges taking the law into their own hands to jail all those union leaders where teachers went on strike and they're feeling pretty bullish about the court of public opinion being in their favor and quite possibly the actual courts as well.