From the article, quoting a law professor. I'd never heard the term "concerted action," so I looked up the relevant text of the National Labor Relations Act. It says that employees have the right to engage in concerted actions "for the purpose of collective bargaining or other mutual aid or protection..."[0]
I am not familiar with this law; could someone explain why employees complaining in a circulated letter falls into this category? I'm (pleasantly!) surprised this was a legally protected action.