Is this for real? How is this even legal?

I didn't know I owed $7 and I received a notice letting me know of the termination of my membership and expulsion from ALPA. That's fair enough. What got me was the 4th paragraph of the letter.

"According to section XX of the current agreement between XXXXXX and ALPA, those pilots who are not members of the Association must pay a service charge equal to the Association's regular dues and assessments".

Isn't that a form of extortion? I have to pay no matter what even after they have expelled me?

Does anyone else see a problem with this?