It's not something that I'd do for various reasons, all of which have been mentioned. The main one is that people, even children, can wait 20 mins till they get through the till, so why wouldn't you? I didn't bring my children up relieving their boredom by giving them food as it's not a good message to teach.
Re: the argument that people may just be starving hungry. Well, a) an adult can simply wait (unless a medical issue) and b) with kids, I tried to avoid supermarket shopping around mealtimes when my kids were younger but if it was near lunch I gave them a snack in the car on the way to the supermarket or if we walked they had one in their pram on the way. Surely no-one takes a small child on a big supermarket shop at a time when they'll be majorly hungry?
Other reasons are that it leaves a mess for others to clean up which is bad mannered and the obvious One is eating food that needs to be weighed, that's just plain stealing in my book.
You wouldn't, say, start putting makeup on going round Boots then put the opened packs on the till.
I do think it's just that people don't want to wait these days. And too many people use food to relieve boredom, or as a habit (expecting food in the cinema, just because "it's the cinema and it's my treat and I can if I want to", despite it being straight after lunchtime and they can't possibly be hungry yet- it's just a great marketing ploy by the cinema chains to get it ingrained in people's heads that you buy their food when you go there.). experts constantly tell us these things are bad for us, and they are. People can disagree if they want and live their life how they want to, but I'm with the nutritionists on this one.