Well, in a rather humorous story, one court in New York has ruled that it may, even over the employer’s objection. In this case, management and the union got into a dispute, but no strike took place because the collective bargaining agreement specifically prohibited strikes from occurring. In order to make its displeasure at management’s conduct known, the union placed a large inflatable rat at the employer’s work site, as is sometimes done in these situations. Read more here.
The employer argued to the court that the use of the rat damages its reputation, but the court held that the union’s interest in the free expression of its displeasure under the First Amendment. The rat did not slow or stop work at the site, and thus was not prohibited by the collective bargaining agreement’s prohibition on striking.