I was a shopkeeper/shop owner for thirty years (technically still am, but that's a different story) and it amazes me that the big supermarket chains didn't clamp down on this abuse when it first became prevalent, fifteen to twenty years ago now. A prominently placed notice 'you must not consume or tamper with goods before they pass into your ownership at the checkout', or words to that effect, would clarify matters. If someone chose to ignore this, or repeated the offence, then banning should be considered at the very least: whether criminal offences took place is a moot point, and not one I'd entertain here. Let's just say, if certain TOCs decided to open supermarkets the magistrates' courts would become even more crowded!