Ah, that must be because of money. Lots of it. They really did officially ban foreign-made cars from entering.
"NASCAR's early history included several foreign manufacturers, such as Aston Martin, Austin-Healey, Citroën, Jaguar, MG, Morgan, Porsche, Renault, and Volkswagen. At a 1954 road race in Linden, New Jersey, Jaguar cars finished first, fourth, fifth and sixth.
As a matter of policy, NASCAR has restricted entry to American car makers since the 1960s, but in 2004, Toyota was allowed to enter the NASCAR Craftsman Truck Series with the Toyota Tundra. The restriction was relaxed in recognition of the fact that the Tundra, while Japanese in origin, is built in America. Commentators have also noted that the "American" cars are often built or assembled in Canada and Mexico."