America is of course a land of immigrants, most of whom bring their own cooking traditions to their new country, and many of whom open eateries or groceries. But it should go without saying that the Old Country food in the New Country will not be the same. There are a number of reasons for this, the primary one being that adaptation is necessary due to the differing habits, tastes and availability of ingredients. An additional factor is that immigrants from Italy to North America were overwhelmingly from Southern Italy, which is why we did not have Northern goodies such as polenta and risotto until fairly recently.
A case in point is that quintessential Italian-American dish, spaghetti with meatballs. Spaghetti of course is Italian, meatballs can be Italian (polpette), but the combination is not seen in Italy. The adaptation was made in the transition from Italy to America because Americans usually have one course (what the Italians call a piatto unico, which is not their norm) and like to include meat. The latter was also scarce in the old tradition of Italian cooking, due to poverty. So that the Italian-Americans solved the problem by putting the two together in one plate. Other instances of this are the various "parm" dishes, which took off from the original eggplant parmigiana (which you will find in Italy) and added meat, for example, in chicken parm (which you will not find in Italy).
Some dishes just are not Italian at all. An example here is Alfredo pasta or sauce, which was apparently concocted by a Roman restaurateur for visiting Americans. And this brings up an important aspect of authenticity. One of the attractions of Italian food is its healthiness, but as you get away from the original by adding more meat, more fats, and bigger portions, you are defeating this quality. A perfect example is American pizza, which often is loaded with meat, cheese and other toppings, and is just too large.
Another reason for the deviation is that food changes in the Old Country, too. Modern Italians are less likely to eat things like the (appropriately-named) offal, which I find awful. Sanguinaccio (blood sausage) may be authentic, but not too many contemporary Italians want it nowadays. And without a doubt (no matter what they tell you) they are also using convenience foods, microwaves and other shortcuts.
To summarize: eat whatever you like without self-consciousness or snobbism, but be aware that if your "Italian" food is heavily Americanized, you will not be getting the health benefits of the Mediterranean diet. You will also be missing the great variety and quality of real Italian cuisine.
