I suspect what we are seeing is a pragmatic, commercial decision by BM members. The members are being faced with a stark commercial choice, either accept the BM policy on this and insist on PCAs, or resign as members.
I think most members want to remain members as it is the professional thing to do, they see membership as an essential feature of being a good quality broker and they will lose business if they cease to be BM members. On the other hand currently they are balancing this against losing business to the non-BM brokers and private sales market and concluding that is a price worth paying, but that might change if large numbers of brokers relinquish BM membership.