Unfortunately as have most of the US carriers.
It's been going on for at least two years now, we had to fight
to find a carrier who were still doing the piece system when
we we're looking at Cancun for cave diving. Alas Delta seemed
to be the last...
We're now given up as it's cheaper and less hassle to go
Thomson direct Manchester-Cancun, even paying for 30Kg
of luggage each.
Not to mention avoiding US Immigration and the gentle
ministrations of the TSA.
I must admit I'd never heard of it being a US 'regulation'
Regards
DaveA