I would be okay with DOT-Rs, yes, but I don't have a decent way to get DOT-R wheels/tires to the event, and I'd rather buy tires I could still reasonably drive on the street. Non DOT approved race compound tires put me into race, go figure.
SCCNH gets such a wide variety of cars that they do their own, very broad, classing system. Otherwise you'd wind up with 3-5 cars in each class. They are also a little speshul when classing cars to begin with, my friend and I co-drove my car and each wound up in two different classes in the results (hand)
As for gutted cars being classed "SM"... well... that's just what I've seen. Not many people who do SCCNH events strike me as super-competitive, most of us are just there for fun (especially the guy who runs a limo with a welded diff), so I don't think many people really care too much how things are classed, as long as it's kinda close-ish.