Due to the times we are in now, more than ever individuals have special dietary needs, not only for medical reasons, but for personal reasons. More than ever people are aware and are sensitive to allergens. As well as afraid of confrontation and being sued do either no choices, lack of choices, or others getting sick. As time goes on, they might feel as if it's best to just let it go instead of having someone lecture on why they brought in certain items and/or realize it isn't so much of a loss to allow people to bring in a couple of sandwiches every now and then.
Disney's policy is essentially, as long as it's not hot, or needed to be hot, it's allowed. I do not how long that policy has been around, but that's essentially what it is. Universal might have decided over time to live by the same thing. As long as you do not bring in a big 'ol cooler you should be good.