I not only think you didn't do anything wrong, I think this is incredibly common. As a matter of fact, my youngest, Outlaw, does it regularly. He's a slow eater, and usually has to bring his breakfast when we take Cowboy to the bus stop, and if I'm hitting the store that morning, breakfast goes as well (sometimes he's still eating when we get home - s l o w eater in the morning:P). Yes, that includes to the toy aisle (both my boys like at least one pass, even if we're not buying. We also have to check out the cakes every time. I let them, because they know not to throw fits over not getting everything they want).
Not only has no-one ever said anything to me, I also regularly see other kids eating in the store. I think you just ran up against a mother who can't be bothered to teach her child that he can't have everything he sees, and thinks everyone else should make sure not to upset her child by giving theirs something he might want. I have to agree with Sharnita, you would have got that response if you were letting LK pick out a toy he wanted, or anything else.