If ALL airlines really do enforce this, here will be the result: instead of checking one large bag, passengers will check two smaller bags. This, of course, will mean that there are many more bags per flight, which will give the baggage handlers more work to do, cause potential delays of departures while these extra bags are loaded / unloaded, the airport baggage systems will be stretched and utilized at higher levels and will increase maintenance costs of those systems, and there will be more of a statistical chance that individual bags will be lost / mis-routed, damaged, etc. Those lost and damaged bags will lead to increased passenger claims for compensation, which will in turn cost the airlines money. This will lead the airlines to discontinue (if legal) compensation for lost bags, and discontinue free delivery to the passenger of bags that are delayed or temporarily misplaced. This entire inevitable situation, therefore, will be met by a few airlines deciding to make the strategic decision to back off of the stringent rules thereby providing thos players with a strategic and marketing advantage. Others, of course, will follow suit if and when they see the net affect is the loss of business.

It's just like attempting to raise fares, and them cancelling that increase when others don't go along.