I think it is because they are moving from a manufacturing base to a service base, and they feel that they need free immigration to sustain their dues. Weird to watch, though. Who would have thought that union bosses would be more concerned about future possible employees than current employees. I see Trump's election as possibly being in part to the union line workers noticing that Trump wants to protect their current jobs, whereas the union bosses are more concerned about perpetuating the union for the sake of itself (and for the sake of perpetuating the union boss jobs--unions have become a sort of industry in and of themselves, and they are trying to survive themselves, with less concern for their current members).