How have the courts interpreted the preemption doctrine in the context of unionization issues?

It appears that the district court adopted the producers' view of the applicability of the preemption doctrine, since it relied heavily on Buckley v. AFTRA, 496 F.2d 305 (2d Cir.), cert. denied, 419 U.S. 1093, 95 S.Ct. 688, 42 L.Ed.2d 687 (1974). We held there that the Garmon rule prevented a first amendment challenge to a requirement of union membership and obedience to union rules, since the matter could be the subject of an unfair labor practice. Although we spoke of "arguably" unfair labor practices, however, the acts complained of were quite clearly subject to 8 of the Act. See id. at 312-13 n.5.