Well I think it is because in other countries people who go to malls and eat have only like 30 minutes to get back to their desks... food courts get so jammed that customers might make a bigger mess if they throw their own thrash because of insane time & jammed pack so better wait for staff people to do their job...which makes some sense..
But here, you've got like 2 hours at least to kill during lunch time to throw that trash in-spite all customers are in line not on the cashier but the trash can..