At many fast-food places, when you buy a soft drink, they give you an empty cup and you go fill it yourself. Do you feel free to refill your cup? Do you think it's theft? (Or both? )

A few places have signs that say either "Free Refills" or, say, "Refills 50 cents". In those cases it's clear. In others it's not. I usually do feel free to refill my cup unless there's a sign explicitly prohibiting it or charging for it. In hundreds of instances, no one has ever challenged me on it. But I still wonder whether the proprietors consider it theft.

What do you think? What do you do?