Disclaimer: They SHOULD have let the SS participate.
BUT...........Just because the SS is registered as a motorcycle in SOME states, doesn't make it one. It's a 3-wheel car! It has a 5-speed manual shift like an auto, roll bar, seat belts, and your passenger sits to your side, as well as being wider than a Corvette in front. All the other vehicles you mentioned being allowed had saddles for seats, passenger sitting to the rear, AND a motorcycle shift system. I'm just sayin! :>)
While I agree with the 3 wheel car statement, it is only our opinion. In the matters of law, it is registered as a motorcycle.... licensed as a motorcycle.... titled as a motorcycle.... and insured as a motorcycle.
As for last sentence, have you ever seen a Stallion? It's about the same size as a Slingshot, and I promise the turn radius is less than the Slingshot. It has a 4 cylinder Ford engine with heater and air conditioning. Not to mention you could buy two new SLR's for the price for one Stallion!