"So?" is being obtuse. That said, I'll take a run at an explanation:
NASCAR TV broadcasts have a global reach.
http://www.nascar.com/international.html
Ford, Chevrolet and Dodge NASCAR race engine blocks are cast offshore, most of them in Brazil. Engine builders source crankshafts, connecting rods and pistons from Germany, Austria and Japan. All 3 manufacturers have plants all over the world. NASCAR is quite simply another extension of the global economy.
A race or two off the North American continent should be on the table for an organization looking to expose itself to a larger audience. Clearly, the base they have is shrinking.
I fail to see how an International event would "just spoil the sport even more".