I agree! Drives me mad! I have read a few articles recently (and replied) in various publications from business travellers (mainly) complaining about "differing standards between airlines as to what is allowed as carry on baggage". As I pointed out, there is a strict limit, printed on the ticket info and also at the desk, and lets face it, regular travellers know.
It is also a problem "from the front" as the weights allowed for pax+cabin bag is not that much, and if everyone comes on loaded up, it affects the weight of the aircraft and so we may end up with incorrect figures.
I think that it should be stressed and perhaps in the "did you pack your bag yourself" selection of questions, should be a question about number of cabin bags and pax should be asked to produce them.
I really feel for all cabin crew who have to deal with those awkward pax who wish to argue about taking their huge/ heavy luggage on as carry on. But you'll get plenty of support from "up front"!