I sometimes see people looking at my cart and I wonder if they are thinking, "Doesn't she know it's cheaper and healthier to cook things yourself?"
A lot of times people are glancing at each others' carts purely out of curiosity. Your life is different from mine, and a glance at your grocery cart is a kind of window into your life without actually intruding on you. I find other peoples' shopping choices interesting, that's all. You can learn a lot about a person from the groceries in their cart.
Other times it's people looking for inspiration. Maybe the combination of things in YOUR cart is a combination that wouldn't have occurred to me ordinarily, but suddenly I've got a great idea for dinner or baking from something I saw you pick up.
Oh sure, people can be judgemental and make unwarranted assumptions, no question. That's why this board exists at all. Some of those people are no doubt making all kinds of assumptions about you, your eating habits, and your morals/ethics based on nothing but the contents of your cart. This is why sometimes I like to put a really interesting combo of things in my cart and covertly watch the expressions on peoples' faces as I unload my Balkan yogurt, three long English cucumbers, pound of butter, and dog collar onto the conveyor belt. They don't know that I'm making sauteed cucumbers with the first two things, plan to bake shortbreads, and my dog has lost weight and needs a new collar before she slips out of the old one on a walk. The looks on SOME peoples' face clearly show that they're thinking all kinds of nasty things about my stuff. This amuses me, but I never claimed that my sense of humor was any more 'normal' than the rest of me.