Why should meat be obligatory to constitute something a sandwich? That seems rather pointless and not smart at all. I'm vegan and the sandwiches I eat don't even contain fake meat, let alone real meat, but they are sure as anything still sandwiches. With or without meat, a sandwich is a sandwich.
No, it is not essential for the Department of Agriculture's definition of a sandwich to require meat. There are many alternatives to meat products that can be on a sandwich. Their definition of a "sandwich" has huge implications for the taxes a restaurant pays, but a "sandwich" can simply be a single topping between two pieces of bread or on a roll. Is a grilled cheese not a sandwich because it does not have meat on it?
A sandwich is defined as some type of filling between two pieces of bread. It is not necessary to have meat as a filling for something to be considered a sandwich. The classic peanut butter and jelly sandwich does not have any meat, but it still has a filling between two slices of bread, making it a sandwich. Vegetarians who make sandwiches without meat are not making something other than a sandwich just because it does not have meat.
The Department of Agriculture's definition of a sandwich does not require meat as an ingredient. Sandwiches for vegans would definitely not contain meat and the Department of Agriculture definitely also caters to vegans, besides it is not necessary that sandwiches in general need to compulsorily contain meat items as long as there are enough vegetables for the filling.
The Department of Agriculture should not change their definition of a sandwich to require meat as an ingredient. A sandwich is simply based upon the fact that there is something edible between two slices of bread. Those two slices of bread define that object as a sandwich. Whatever comes before the word "sandwich" is what further defines the ingredients of said sandwich. For example, if you want your sandwich to contain ham and cheese, then you call it a "Ham and Cheese Sandwich". If you want your sandwich to contain cucumbers, then you call it a "Cucumber Sandwich". Whether there's meat inside the sandwich or not should not matter.