Hot dogs were first introduced to America in the 1800's, in New York's Coney Island. As one of their first nicknames, they were referred to as both "Coney Island Sandwiches" and the "Frankfurter Sandwiches".  Perhaps this does not apply to the original creators of the food item, the Germans, but it certainly applies to America. The hot dog was called a sandwich in its original glory days, and thus still must fall under its original category. It doesn't make sense to change the categorization of a product just because people are stubborn in opinion.
As mentioned, the hot dog is not considered a sandwich EVERYWHERE. If it's not a universal thing, why should every single hot dog be categorized as a hot dog? I f everyone can't agree on it, why place a controversial label upon it?
[P1] Hot dogs were first called sandwiches. [P2] Hot dogs should remain under the sandwich category.