It depends on what you think manners are for - whether you regard them as useful at putting others at ease and making their lives more pleasant, or whether you secretly feel that manners are really a sort of signalling system, primarily for demarcating class lines.
If your notion of manners allows you to regard other people as lesser than yourself because they behave in a lawful way that you dislike, then other people will judge you for that.
Personally, I don't let my older child eat things before paying in shops because I am trying to teach him not to touch them at all or to fiddle with things he's not paid for, and it would undermine that. And I don't do it myself, because I was brought up to think it was rude, yes. But other people were brought up differently to me, and what baffles me in this thread is rigid certainty that our own individual upbringing is, or at least should be, the universal gold standard of behaviour. Why does it matter what others do in this trivial instance? Why the interest or concern? It's not theft, it's not smelly or loud or obscene in any way. It's just other people living a very little differently, with slightly altered perceptions of what it is to be mannerly. Why is that grounds for such judgement? And to attack people for that in such personal and nasty terms, and all in the name of being polite and well-mannered! I mean, seriously?
I think those who provide snacks for very tiny children and babies, so they don't cause annoyance to others, are being thoughtful and considerate, which is the very essence of politeness. And to those who say snacks should be brought into the shop from home... has it never occurred that people buy food from the same place, and that it may put a poor cashier in the awkward position of challenging non-payment for such a previously purchased item, and a parent in the position of explaining that scenario and hoping it doesn't sound like a lie? Instead of just buying the damn thing there and then?
And anyway, how are you all so certain that the snacks thus offending you haven't been brought from home? How do you know? Have you asked?