My reasoning is that a hotdog is a sausage. When you say you want a sandwich, you don't say "pass me a ham" you say "pass me a ham sandwich." When ordering a named sandwich, "I'll have a Ruben" it's widely understood that a Ruben is a sandwich so the modifier is already packaged in the name. A sandwich has "Sandwich" as a defining modifier.
When you ask for a hotdog you don't say, "give me a hotdog sandwich" you say, "give me a hotdog." The same situation works with bratwurst, you don't order a brat sandwich. To further reinforce this, if you're in the south and central US and order a Hotlink it comes on it's own or in a hotdog bun but if you order a "hotlink sandwich" you get two hotlinks cut length wise and placed on a hamburger bun or bread.
A sausage can have a bun as a condiment and still be just a sausage. A sandwich can have sausage, but is still refered to as a sandwich. So a hotdog is a sausage served with bread, not a sandwich.