Sure - in fact, I find a union to be a real free-market sort of concept when taken in its purest form.
It boils down to "treat us better or we all quit, gl replacing your entire workforce". If the employer calls then on it, that's a strike - employer either has to cut a deal or try to replace their entire workforce. In that way, it regulates employment without any interference from government.
Of course reality isn't so cut and dry, but theconceptt of a union isn't really communist at all IMO.