The Fiat 500 and 600 are exempt from US regulations because they were over twenty-five years old when imported. The Mercedes-Benz and the Chevrolet Astra must have slipped through the net, which seems to happen sometimes. I once read about a Peugeot 206 cabriolet which was used to smuggle drugs from Mexico to the US. It was seized by the DEA and put up for auction, and in the process managed to acquire Texas (I think) license plates. I suppose that it got through because it never went through the official importation process and so was never examined to determine whether it complied with US regulations.