They can walk out WITHOUT their food/purchase. How many places hand you your purchase BEFORE paying for it?
Why is this so hard?
The transaction was payment for a meal already eaten. At that point it is sort of hard to "walk out without their food". I guess they could stick a finger down their throat and barf it back up, but I would suspect the mechant "might" not want it back in that condition.
See, it's not hard at all.