Some, but not all, of the other products are marketed as medical products to doctors for use in hospital settings. There's a bunch of regulation over those products.

EDIT after downvotes. For fucks sake, it's pretty obvious that a product making medical claims to doctors has more stringent regulation than food.

Here's the FSA guidance to industry about enteral food products: http://www.fda.gov/Food/GuidanceRegulation/GuidanceDocuments...

To find that page I put "Does the FDA regulate enteral food products?" into a search engine. This is the first result. (Although my filter bubble might affect that.)