ALPA is not a union. It's an association of twenty-something independent unions. Each individual MEC uses the resources of the association at their discretion for their own purposes. Once you understand that, you'll understand the difference.

But ALPA was created, first, for the betterment of safety. If all airline pilots were paying dues under one association (or union), they would have more leverage in the halls of congress, the White House, the judiciary, the nmb, TSA, dhs, fbi, etc. ALPA is, first and foremost, an aviation safety advocate. And we should all feel compelled to improve safety first. We do that by uniting under one association/Union. The bargaining and job protections should be secondary and tertiary.