The fact that they didn't invent the term doesn't by itself make a difference; borrowed words like "Apple" are sound trademarks just as coined words like "Kodak" are.
What matters is that there's a good argument that "Footlong" is merely descriptive: it describes a quality of their sandwiches that is no different from any other sandwich made from a 12-inch roll. Descriptive terms can't be trademarked unless they've become entrenched or famous, such as "Bank of America".