Wilson most certainly is American born. The company was founded based on a New York meatpacking company, although most of its historical operations were based in Chicago. It took the name Wilson in 1915 and produced gut strings and sports uniforms, among other things. (Gut strings were one of the main conduits for their transition from meatpacking to sporting goods.)
It only became a holding of Finnish-based Amer Group in 1989. Amer has quite a few sporting goods holding in various countries (including the very American Atomic Aquatics), and they still allow Wilson to run most of their operations out of the US. (Amer is the largest sporting goods holding company in the world.) However, as with the majority of racquet manufacturers, the frames are made in China these days.
As for various posters' sentiments on this thread about loyalty, etc., that's a pretty maudlin way of looking at tennis racquet contracts. For years, Pete played with Wilson, and he stuck with them primarily because he didn't want to switch frames and was superstitious about his equipment. His unwillingness to use current models or paintjobs cost him quite a bit in endorsement money over the years, and there was no reason to expect loyalty between him and Wilson from either side. Pete resented that Wilson didn't pay him more, and Wilson didn't like the fact that he wouldn't at least paint his frames to look like a current model. Racquet contracts are a financial arrangement, and Wilson did their best to pay him as little as possible, since he didn't want to switch frames. Pete's agent did his best to get him as much money as possible, given the circumstances, and the limited value of his endorsement. In a nutshell, it's a business arrangement, not a friendship, marriage or magic spell. The idea that Wilson would unnecessarily give him a lifetime contract as a way of saying "thanks" is ludicrous. Similarly, the idea that Pete would keep using a frame that wasn't of his choosing any more just out of loyalty to a brand, and without being paid is also ridiculous. I think everyone has been watching too many romantic films.
Oh, and by the way, it's a Pure Storm Tour GT.