According to the report, the agreement says that overtime isn't voluntary if there are too few volunteers to do the work. Definitions of "voluntary" are beside

According to the report, the agreement says that overtime isn't voluntary if there are too few volunteers to do the work. Definitions of "voluntary" are beside the point. Also, the report says part of the employer's complaint is that there was no vote taken on the action. The implication is that had the action been official, i.e. called by the union following a vote, then the threatening letter would not have been sent. It looks to me like the employer's letter was inadvisable, and perhaps should have been reviewed by the company's PR team before it went out.