I'd say the burger shop can safely assume that the buyer won't screw them over an $5 order. thus they're able to release that burger after seeing "0 confirmation" transaction appear on their client.
They may assume that, but from your explanation, you make it sound like the buyer could conceivably screw them over, which guarantees that a group of individuals will exploit this to maximum benefit, which then means vendors cannot assume that. Maybe burger joints wouldn't be the target of said evil individuals on regular basis, but it's not clear to me that the mechanism is ready for prime time.