I knew someone who would microwave Totinos. Most disgusting thing I've ever seen.... but she was hot so I only halfway judged her for it. lol
Did you bring your own dishes? Did you choose to sit at a server's table who could've had a customer wanting an actual menu item if you didn't plop your butt down?
Think of it like this, did you make the pizza? No. But why do we not tip the cooks? Because they are paid a real wage. That's the difference. If the person behind the counter is paid from the business like they get tips to, then tip them. If the person ringing you up makes minimum wage or better, then don't. I tip anyway, because **** it, its just a dollar or two.