First, what Grim said. Secondly, some few items (and I expect there to be more instances of that in the future) are food, but also have other fucntions, like the canteen or alcohol. Filtering out the 'pure' food items gets annoying the more combined items are added.

Third, this would require altering Bobby Ray, Map Item creation, AI inventory creation, shopkeeper and mercenary inventory code. As soon as this would be added, people would then demand other items to be directly tied to specific features. And I don't see why the code should filter out items (which has to be constantly maintained for every new capability) when modders can just alter the xmls and have items they do not want not appear. That's why we have xmls in the first place