Dear Huan Yi,
Possibly for the same reasons that the Pakistanis got rid of their last President, so that now their new one must balance being pro-American on the one hand (his enemies, the guys in the Pakistani intelligence services and the hard-line Islamic extremists accuse him of being pro-American, and can hardly wait for him to show some overt sign of it to justify a revolution) and being a Pakistani patriot (and making sure that his country's borders are respected by what the extremists over there still think of as The Great Satan).
Pretty much the same set of conflicts that any national leader has to face.
Except that his country is pretty much in a state of near revolution all the time and his status is very uneasy. His wife, who won the office, was just recently murdered, if you remember, and if he's too supportive of western interests, not only will those interests not be served, and his people not gain greater freedom, which I believe he does want, but his country will be thrown back into the camp of the Islamic extremists and, oh yes, by the way, it's likely he'll be assassinated as well.
The United States has been free to do this sort of thing all along, Huan Yi. To insult a guy who's trying to help us out while the head of our own Joint Chiefs of Staff is in country to provide reassurance and stability raises the same question you just asked me, and, as well, anybody else here who's interested.
Why are we doing it while the head of the JCS is in country? Is it for the oil? Or, perhaps, is Cheney looking for another war to start now that Iran looks as though it may not fly apart as he's been working for over the past few years.