The APSA is crossing picket lines for its Labor Day meeting because, well, it's just inconvenient not to.
Let me point out the obvious--your criticism doesn’t help Unions. It just means that more and more groups will intentionally hold conferences in cities that are not unionized. An association can’t just cancel an annual meeting; instead they will insulate themselves from criticism by blacklisting unionized cities--or hotels with unionized workers.
Are you implying there are picket lines at the Convention Center that will need to be crossed as well as the hotels?