All people in the United States are entitled to the civil rights protections of the US Constitution. Pursuant with the Supreme Court's decision in Reid v. Covert, 354 U.S. 1 (1957), the Constitution and the civil rights provided therein supersede any treaty with Italy. The 5th Amendment to the US Constitution states in part: "nor shall any person be subject for the same offense to be twice put in jeopardy of life or limb;"
Amanda Knox was convicted in 2007, acquitted in 2011, and now again convicted by the Italian courts. While in Italy she was subject to Italian law. Amanda Knox now resides in the United States and is subject to the laws and civil rights of the United States, not those of Italy. Those rights, granted in the Constitution, supersede any extradition treaty with Italy.