Italian sausage in the United States is generally identified as containing fennel just as the aforementioned Wikipedia article stated. If it were not so, U.S. pizza operators who offer both sausage and Italian sausage would be in a true marketing fix. How else is a consumer supposed to know the difference between "sausage" and "Italian sausage" when the "Italian sausage" might not contain fennel? Many chains offer both. The one I worked at many years ago certainly did, and the difference was unmistakably fennel.
Perhaps those of you who don't believe that the sausage nominally known as "Italian" contains fennel could explain what consumers are to look for and expect in the differences between said sausages when they order a pizza.
I italicize the term "nominally" above to emphasize that what we're talking about is a nominal classification, not taxonomic. In the same way a "two-by-four" is not actually 2" x 4".