This case is not even union related. Video at link. Even public info about union participation can not be placed in your file. You also can't watch an employee going to a union meeting in a public place etc. It is illegal period.
http://abcnews.go.com/Business/facebook-firing-labor-board-takes-stand/story?id=12099395Labor Board Says Firing Employee Over Facebook Comments May Be Unfair, Company's Internet Policy Too Broad
By SUSANNA KIM
Nov. 10, 2010
For the first time ever, the National Labor Relations Board issued a complaint that an employer engaged in unfair labor practices for firing an employee who made derogatory posts about her supervisor on Facebook.
The labor board issued the complaint against American Medical Response of Connecticut last week for firing medical technician Dawnmarie Souza after she criticized her supervisor online. The board also said that the firm had an overly broad employee Internet policy.
Images of Souza's Facebook page, provided by a lawyer for the company, show remarks including, "looks like I'm getting some time off. love how the company allows a 17 to become a supervisor," using the company's terminology for a psychiatric patient. Another post says, the supervisor is "being a d***" and a "scum***."
Calls to Souza were not immediately returned.