Because they are lazy and have cups in their car left over from either earlier in the day or previous days, and since theres no garbage there they toss them out.

The argument some people have here, "they used to have garbages, but have removed them, so litter away! " seeminlgy justifying this behvaiour in an odd pro-litter protest is crazy.

Almost every other fast-food restaurant with take out windows has 0 garbage bins, but you don't see their trash at the order kiosk.