When I visit the States I see a lot of pasta and spaghetti in the supermarkets. I wonder which is the typical seasoning (sauce or similar) used by Americans when they cook spaghetti. Of course, when they don't cook Italian-style. 
The most common preparation is tomato sauce and parmesan cheese.  Of course, some Americans will tell you that the only truly American way to eat spaghetti is to top it with chili, then shredded cheddar cheese, then chopped onions, then red kidney beans.  It's hard to fully express the majesty of this dish, and should only be enjoyed on special occasions, like the Super Bowl.  In Canada (which I consider to be basically "American"), it's also fairly common to add or replace the tomato sauce with pesto sauce.  Soft goat cheese is another common additive.  Perhaps these sound "Italian" but I'd consider an Italian pasta to be something more like a bolognese, carbonara, alfredo, or even the trusty old butter/olive oil and parmesan cheese.  You'll almost never see tomato sauce combined with pesto sauce in an authentic Italian restaurant (or in Italy), but it's common in American home cooking. 