Another reason might liability. Many non-SCCA clubs use SCCA rules to one extent or another, but under differently controlled circumstances. If a serious injury occurs and someone claims it was "because" of the SCCA rule, and that rule was published with no connection to a signed event waiver, SCCA could possibly face a serious lawsuit. Frankly, I'm a bit surprised that they'll even sell the rulebooks to non-members, but maybe that's a mistake due to the old profit motive.