If things weren't closed on sunday, when would anyone get a break? Don't be so ignorant as to forget about the guy behind the gas station cashier, or the man who sells you groceries? Hell, how would you like it if you had to go to school 6 days a week instead of 5? Would you enjoy that? I didn't think so.
Besides, Anthony summed it up, it's an understood off-day. Live with it.