Agency Shop
Agency Shop really doesn't mean that you are a member "like it or not". If you do not want to be a union member that is just fine, however, under agency shop language you are still required to pay your share in the form of dues.
You can choose to not be a union member, but you still work under the work rules established by the union. To prevent a "free ride" situation you pay dues without the requirement of union membership.
As for the "grandfathered" pilots, I know a lot of carriers have them and I have never heard of them turning down a raise that union volunteers busted their arses for...yet they refuse to pay 1.9%. Pretty pathetic in my book.