Those firms are registered with the SEC. But they chose to be regulated by the CBOE, not FINRA. They are referred to as "non-FINRA BD's" and are perfectly legal as long as they don't interact or solicit with the public.
Quote from bulllives:
what about Prop firms opening LLC sub accounts thru CBOE firms? Are they really registered with the SEC that way? Or is this just another loophole to bypass FINRA registration?