These are the kinds of stories we hear all the time, the one that you experienced. I think paragraph 8(5)(c) of the Income Tax Act talks about ordinary operating expenses of the union. Tax court case law, in a CRA interpretation bulletin, says a union that gives money to the Vancouver International Film Festival isn't spending it on normal operating expenses and, therefore, that those dues aren't tax deductible.
On May 2nd, 2016. See this statement in context.