The Porsche 959 was not submitted for crash testing, hence why it was not allowed onto American soil. Even though 30 "US-Spec" 959s were built, only 8 were delivered to America before the government stepped in and had transfers stopped and the 8 cars impounded. Porsche had tried to import the cars as "race cars" (meeting different standards and needing no crash test), but people caught on.
7 of the initial 8 were sent back to Germany, and the other 22 were sold off to European buyers. The lone remaining US-Spec 959 was kept in a showroom, not allowed to be driven or even raced. Another 959 was bought and modified to a true race car and brought in that way. Others have been brought in through illegal deals while others have not made it past the US Docks (See: Bill Gates' and Paul Allen's 959s).
Canpera Design of California stepped in and entered into a legal battle with the government which led to the creation of a new law. This law states that any car which has a production of less then 500, is no longer built, was never sold in the US, and they were considered "rare", that they did not have to pass DOT crash tests and thus could be imported to the US only if they passed EPA tests and were driven less then 2,500 miles a year.
To make DOT, the government, and insurance companies happy all owners had to sign agreements that they would not surpass the 2,500 mile limit.
At the writing of AutoWeek's article on this in 2003, 3 had been legalized and 12 were on the waiting list. 10 more had been legalized by other companies but were likely to be finalized by Canpera since he not only makes them EPA legal, but modifies them as well for even more horsepower.
The McLaren F1 on the other hand has always been US legal since it was DOT crash approved. McLaren F1 #XP2 was sacraficed for that.