Sure they will open for other things but a VAST majority of their sales potential is going down the drain. Sundays bring a packed house guaranteed 8 times a year or more (hopefully only eight).
They close on Sundays for religious reasons as much as family values. They have also openly criticized people whose "values" don't meet their political, social, and religious ideals.
If they want to make a statement about their values and sacrifice that much capitol I do however respect that. I just think it's dumb.
If they care so much give the extra hours to the single moms and dads who don't suck up welfare and can really use the money. Or donate it.
These social, religious, and family value thing is all part of their marketing strategy. Nothing more