Quite a few years ago, here in Oregon, there was a association in another sport that "prohibited" its members from taking assignments from other associations. One of its members threatened to sue, claiming that made him an "employee", not an independent contractor. The State Bureau of Labor and Industries investigated and agreed. The association recinded its restriction to avoid fines and possible civil suits.
IANAL, but it's my understanding that the only time an association can restrict you from accepting work from someone else (without you being an employee) is if the other work would be detrimental to your performance with the first group and/or would be "harmful" to the first group. And - this has to be a stated policy and condition of your acceptance of their assignments.
Or maybe not.