Yes, they just have to be accountable. They have to be accountable for their political activities and all other activities, because of the generous tax treatment that they receive, and also because of their forced funding model.
Some have said that what's good for them is good for everybody. I say that what is good for them would be great for me. If the Government of Canada were to propose some sort of Rand formula so that every open shop construction company had to be a member of Merit Canada, my revenue would go from a few hundred thousand to potentially billions, just like the trade union movement, and I would happily submit to increased transparency. I actually think it would be my moral obligation to do so, and I wouldn't be here fighting it as they are.