What shop do you know of where they'll let you choose food, take a bite and then pay what you feel it's worth? Hmm, this donut's a bit stale, I'm not prepared to pay anything for it thanks Mr.Tesco.
Sometimes I buy fruit and it goes bad too fast. That's life. Sometimes you get a shitty waiter, that's life, but now you get to unilaterally decide you'll get that service for free? Because there's a significant minority of people who will always find fault just so that they don't have to pay.