I think - with any issue of discipline (meaning teaching), there is context. Willful defiance is one thing, but inability to cope is another. Any kid who is very young, tired, genuinely hungry, has sensory issues or otherwise - that's not their fault, it's not within their ability to control yet. Note that I mentioned 'young' - for those of you with older children, obviously you know what they're capable of and parent accordingly.
In terms of ethics though (assuming it's not illegal and stores are ambivalent on the issue) - it seems clear that there's not an obvious moral absolute here - it's a personal decision about whether you feel it's right or wrong, and also circumstances come into play - what works for your family. No point judging because someone's child has sensory issues (and you won't be able to tell unless they have it tattooed on their forehead), or because the parent is a bit disorganised/unprepared (despite perhaps being a brilliant parent in every other area), or for whatever other reason. Obviously the people feeding their kids in supermarkets aren't doing it to be morally deficient people who bring up morally deficient children - they have their reasons, and who am I to judge the quality of those!