My understanding is that heavier luggage requires special handling (and is thus more expensive) due to concern for safety of the people who handle them.
Well again its an arbitrary distinction, Is that 51 lb. bag that much more dangerous than the 50 lb. bag. A baggage handler could easily throw their back out improperly lifting a 49 lb. bag, but at least I won't get charged for it.
It is kind of arbitrary, but unless your going to eliminate weight limits altogether, I don't really see how else you could do it.