The problem is that the rules change depending on the operating carrier. Each operating carrier writes its own OpSpecs with slightly different variations in the rules, and those OpSpecs effectively become regulatory once the FAA approves them. The net result is that the rules may be different from one RJ to another, and those rules are different from the rules for mainline. I have no doubt that in many cases FAs are making up rules, but you really have no way to know whether or not that is the case, especially given that enforcement of said rules is inconsistent even within a carrier.