Is it a bit quirky for you to be in a line to order and the person in front of you, an adult, orders the kids meal. Then when you suppose that its for their kid you see them go sit down at a table and eat it and pocket the toy.
That could be a reason but if they are worried about how they are eating then junk food isn't the way to go. Plus a kids meal is not the cheapest thing on the menu. I guess this would be different too if they started to play with the toy.