There is a difference between what you must declare and what is forbidden. Just because you have to declare it, it doesn't mean they will confiscate it. Agriculture may want to inspect it, and to determine if it is acceptable.

The proper thing to do is declare your food - 99 times out of a hundred (maybe more), the primary inspector will ask you what it is, and most things, chocolate, candy, cookies and about 95% of the stuff people actually bring - stuff other than meat, fruits, veggies, dairy, seeds, nuts, etc. - they will send you on your way. If you have any of those other things, they'll send you to be inspected, and you still may be ok.

The fact is - the US form does list the generic item "food" as one that must be declared (unlike most other countries, which list only specific kinds of foods). It is kind of sneaky the way they list it, because it is in the middle of a list of specific kinds of foods, so it is kind of hard to notice unless you are paying closer attention, but it is there in no uncertain terms. I live in the US, and when I used to bring cookies from home in Canada, etc. I would always have to declare "food," however, when asked what it was, I told them, and was always sent on my way. Going back to Canada, these items wouldn't need to be declared.

I once spoke with a customs officer at length about the food items that are not allowed into the US, and in no uncertain terms, he said that the only main categories of things that are 100% forbidden are citrus fruits (even those originating in the US) and goat meat. Other things, for example, fruits and vegetables may not be permitted based on something like country of origin (or if that can't be determined), but nothing else is outright. An apple, to use something that has been brought up in this thread, is not necessarily outright banned, if inspected by agriculture and determined to be from a place on a list of accepted countries (and dependent on where you are arriving from). However, my guess is, most of the time, simply not declaring your apple will lead to its confiscation if found, no inspection, because you failed to declare it.

One thing to note - may not be applicable to many on this board - but customs does seem to be loosening on this a bit, at least with trusted travelers. Those enrolled in Global Entry, and who use the kiosk's to enter instead of the paper customs forms, do not get asked to declare the generic "food," and are only asked to declare specific types of food, like most other countries I have been to do.