Although technically grazing isn't shoplifting in legal terms, it is a sackable offence if you're staff and the simple fact is it does costs stores hundreds of millions every year, because so many people
are dishonest and don't pay for what they eat (apparently everyone in this thread is an exception), thus they have a no grazing policy which they can enforce. The reason most shop floor staff won't challenge grazers is because it usually ends with said worker receiving a volley of verbal abuse or if you're at the till the last thing you want to do is go and make a scene about it.
They have a single blanket rule for a reason. Firstly because if you allow it, where do you draw the line, both with what people consume and how much they consume? If it's okay to walk around eating a sandwich, is it okay for me to
make one? What if I go in, eat about 10 different things and present the cashier with nothing but empty packaging? Would I only do this at a big supermarket or is this acceptable at a newsagents or corner shop?
Additionally, because it's hard enough to spot people grazing and not paying for items, relaxing the rules would make it even easier for people to steal. Grazing and not paying is much easier to get away with than stuffing items into your clothes or bag, because while the former will only draw the ire of some of the public, the latter is usually reported by other customers to staff because it's obvious they're trying to steal. The more people eating on the shop floor, the easier it'd be for dishonest people to blend in and the lower the chance of being confronted about it/getting caught. They already lose enough money from it, it'd be mad to encourage behaviour that would increase lost revenue, they already get rinsed by traditional shoplifters as it is.
http://www.which.co.uk/news/2007/12/britons-admit-to-stealing-at-self-scan-tills-127519/
They don't have 'items must be paid for before being consumed' signs up just to be spoilsports, thieving is a real problem in this country. Walking round eating stuff makes it easier for those doing the same thing who don't intend to pay to get away with it.
You're enablers!
P.S. where's Gordy!?