The legal definition of theft - at least where I live - is all about intention. It involves an intention to deprive another of their property. No intention, no theft. If you absent-mindedly walk out of a store without paying for something, no theft has occurred. When our kids were babies, we used to put the shopping in the pram. One day I left the supermarket and down the street discovered a loaf of bread in a different section of it, that I’d forgotten to pay for. I went back and explained myself to the security guard, did he call the police? No, he commended me for my honesty, and let me pay for it with the self-serve checkouts.

For a supermarket, their biggest concern with theft is the repeat offenders. If it is an unclear situation, it is in their best interest to give the customer the benefit of the doubt. But, if the same unclear situation happens again and again, that’s when the intent (which is legally required to constitute stealing) becomes obvious. Ultimately though, it is up to the store staff, police, prosecutors and magistrates to apply a bit of common sense in deciding what is likely to be intentional and what likely isn’t. But yes, given theft is defined in terms of inferring people’s intentions, “zero tolerance” is a concept of questionable meaningfulness in that context.