"PurplePeony, if you are still there... I am with you one hundred percent. I can't think of a single good reason why a child needs to be fed in a supermarket at all, never mind with food that hasn't been paid for. It just gives them the idea that a) they don't have to wait for what they want, and b) that food is the solution to any kind of negative feeling. Sorry, but the moral high ground (terribly unfashionable, I know, but there it is) is with people who pay first and eat later. The rest of you know that you really don't have a moral leg to stand on, hence all the 'witty' comments."
So true.
This would be a fascinating and insightful response to the OP if, indeed, it answered the OP.
However, what the OP actually asked is not, "is there any need to do this?" but "why do people do this?" And the answer is clear. Because they want to. Because they have small children who are hungry and whom they are providing, not with Easter eggs and Penguins, but with a small amount of food to tide them over. Because the supermarkets - who own the food - do not object as long as the food is subsequently paid for.
It does NOT give children the idea that they do not have to wait for anything - it gives them the idea that they do not have to wait for food; which, in the absence of a compelling reason (none of which - spoiled appetite; social occasion of a forthcoming meal - seem to be under discussion here) is actually a pretty good way to think unless you are a PoW.
It does NOT give children the impression that food is the answer to any negative feeling - it gives them the impression that food is the answer to hunger. Which, in sentient aerobic organisms, it is. Again, a pretty good lesson to learn.
Anyone who wants the moral high ground is welcome to it. Which supermarkets do they have up there, by the way?