I'm no expert on labor law, we leave that to Matt, but one pays attention when the decision of an Administrative Law Judge makes the paper. You can't fire workers for trying to unionize, and I really don't understand WalMart's defense of its decision to fire workers who tried to unionize.
Most of the allegations related to a coordinated set of strikes collectively referred to the "Ride for Respect" because they involved traveling by bus to the company's headquarters in Arkansas for protests at its shareholders' meeting in June 2013.
WalMart called this an unexcused absence and disciplined or terminated the workers who went on the Ride. To me it looks like an effort to unionize, and I didn't think that was a legal firing offense. But it sounds like the firm prefers to fire workers who try to unionize and face the consequences.