In addition to being the spiritual head of the Catholic Church, the pope is the temporal head of Vatican City. In other words, he is a head of state. The U.S. president and the queen of England have fully armed security details, so it should not be a surprise that the Pope does as well. Do not forget that John Paul II survived an assassination attempt in 1981. It makes sense that the Vatican would seek to prevent anyone else from trying again to harm a pope.