I'm not a labor lawyer, so this article doesn't condone anything or give legal advice. You should double check everything here.
What's a ULP?
You see it on the bottom of some of the signs: “ULP Strike”.
ULP means “unfair labor practice,” and refers to any actions by employers or unions that violate the National Labor Relations Act (NLRA).
The biggest surprise, to me, was that ULPs can be charged against unions as well as employers. This is because the NRLA defines rights and obligations for both sides, and if one side fails to fulfill its obligations, it's a ULP.
The most prominent ULP that's being used by OUR Walmart and the Fight for Fifteen fast food worker organizations are those involving retaliation by management. They are saying, particularly in the press, that Walmart is retaliating against activists by firing them.
Walmart, also in the press, is saying that the activists are being fired for not fulfilling attendance obligations.
A ULP on the part of a union is trickier. One example given on the Federal Labor Relations Authority is if the union collaborates with the employer to punish a worker who didn't join the union. That's obviously not going to happen at Walmart, but it could happen in situations where labor is friendly with management.
What's a ULP Strike?
There are two broad classes of legal strikes, “economic” and “ULP”. ULP strikers have considerable protection, because they cannot be fired or permanently replaced. When the strike ends, the workers are entitled to get their jobs back.
Legal strikes? Are there illegal strikes?
Yessiree bob. If you have a strong union, and want to punish “scabs”, you cannot have a strike demanding that the boss punish, fire, or demote the scabs. It's a ULP to get the employer to punish the scab. Any strike with a goal of committing a ULP is categorically illegal.
Read the link above for more education.