According to the NHTSA, these are cars that have been denied a request in the USA for import under Show and Display for one reason or another:

2010 Alfa Romeo 8c Spider Roadster
1993 Aston Martin Virage Volante
1989 BMW M3 Cabriolet
1990 BMW M3 Sport Evolution III
1993 Jaguar XJ220S
1993 Lamborghini Strosek Diablo
1995 Nissan Skyline GTS-T
1998 Nissan Skyline GT-R Autech Version 40th Anniversary Edition
1989-1994 Nissan Skyline R32 GT-R
1989 Porsche 959S 1990 Porsche 962 RLR
1992 Porsche 964 RS
1990-91 Nissan Figaro
2008 Fiat 500 Abarth

But here is the non-shocking non-surprise, the US govt in it's typical screw Americans way of doing business and failing at everything, is just fine with Canadians and Mexicans bringing them into the USA and enjoying them for up to a year, so long as they don't sell them to Americans.

Or wait until the vehicles are at least 25 years old

Full list of the rest of them at http://blog.dupontregistry.com/features/2014-foreign-cars-that-are-illegal-in-the-usa/