I think there's legal issues there; a place, by law, has to make sure you're making at least minimum wage. If your hourly wages plus tips don't equal that, then yeah, they have to make it up to you. Or at least, that was my experience as a server. It took me a bit to figure out why I had hostess wages when I hadn't hosted during those two weeks.
As for Sonic carhops...I had a roommate who was a carhop and then managed. He used to make awesome tips--equaling more than he made as a manager!--because he's just that great a guy. I mean, they are bringing the stuff out to your car. I always tip them because I hated getting cheap tips or no tips as a server. Sympathy costs me.
Threaded Mode