Bringing in PB sandwiches or other non-perishable snacks -- if Disney doesn't care enough to stop it, then it doesn't matter to me at all -- to paraphrase Thomas Jefferson, it neither picks my pocket nor breaks my leg

Kind of like people bringing candy into the movie theater -- why would I care?
Perishable items? Well...I suppose it's always possible that they have those reusable ice packs in their backpack, like the kind you put in insulated lunch bags.
Canned food? Hmmm...well, cans are sure heavy to carry around, so it seems like there would be an easier container to bring in. I could see maybe eating canned fruit (not at WDW, maybe elsewhere), but not cream of mushroom soup, ewww. Some of the things people have mentioned here -- the soup, cold Spam, canned corn -- I don't think it has anything to do with finances, it's not the cheapest stuff you could eat, it's just weird. People would think it was weird if you ate that stuff for lunch at work or school, or if they went to your house and you were eating it.
And the calculator? I wouldn't take out a calculator, but if what we ordered was considerably less than other members of the party -- unless we wanted to specially treat them (which is certainly possible), then I'd only want to pay for what I ordered, and I'd estimate, add in tax and tip, and round up. Especially if others have ordered drinks or wine, their tab might be a LOT higher than ours. I agree with allyphoe
Changing dirty diapers in public, especially in a restaurant? ICK! Virtually every restroom I saw at WDW had a nice area for changing babies -- why would you do that where people are
eating??? I mean...I would certainly hope that the diaper-changer was planning to wash their hands after their task...another reason that the baby should be changed in the bathroom, near a sink!
