The American Psychological Association (APA) is a nonprofit organization, so it can't use membership dues for lobbying. That's why the APA established a subsidiary, the APA Practice Organization (APAPO), to engage in the lobbying that the APA can't. Of course, the APA still can't shuttle any part of its membership dues into the APAPO, so it came up with a new tactic: including a line item for a separate "special assessment fee" in its members' dues statements. You can see where this is going: The special assessment fee doesn't go to the APA; instead, it goes to the APAPO.
APA members discovered that they actually couldn't be required to pay this special fee, even though their dues statements never said they didn't have to pay it. Several members sued under unjust enrichment and false advertising. The district court granted the APA's motions to dismiss. This appeal to the D.C. Circuit followed.