For me it's a pain in the butt to take a car in, let alone do it twice or more. If you go through the mcdonalds drive thru and get home and the order is wrong, even they usually try to make it up to you.
Maybe they count on you not being willing to go through the hasslr of bringing it back. I guess when you see that many people you can't please them all.

Good luck with your situation, maybe you should keep taking it back, if you can do that, and they'll try to make it up to you somehow.

Dh