So looks like you're right but there's also some weird language technicality for "closed shop" where it's really a "union shop".
Per the APFA contract[1] employees are forced to join the union within 60 days of assignment as a flight attendant. This is technically considered a union shop (not a closed shop) because it doesn't require people to be union members before being hired.
Under the Taft-Hartley Act a lot of states (and in some situations, court decisions) have made this illegal[2] via right-to-work laws but airlines are covered under the Railway Labor Act (45 U.S.C. § 152)[3] which allows it (upheld by the US Supreme Court in Railway Employes' Department v. Hanson, 351 U.S. 225)[4] .
So, I was wrong and the employees had no choice but continue to be union members and pay dues or be fired because of airline-specific labor law.
