so they had probably just come across the border for that. But as well as that, there have been American troops stationed in Pakistan in the past few years:
By March 2004 there were reports increased US operations in Pakistan. Two air bases -- Dalbandin and Shahbaz -- were the focus for extensive movements to provide logistical support for special forces and intelligence operations. Shahbaz Air Base near Jacobabad appeared to be the key to the US’s spring offensive. At Jacobabad, C-17 transports were reportedly involved in daily deliveries of supplies. A report in the Pakistani newspaper the Daily Times <10 March 2004> claimed that the airbase was under US control, with an inner ring of facilities off-limits to Pakistan's military.
http://www.globalsecurity.org/military/facility/jacobabad.htmand it seems they're still there in some form:
The debate actually started when PAC Chairman Malik Allah Yar Khan forced Defence Secretary Lt Gen (Retd) Tariq Wasim Ghazi to concede, contrary to his July 2006 statement before the same PAC, that the US-led allied forces stationed in Afghanistan are still using Pakistani air bases to carry out standby operations in the ongoing war on terror. He informed the PAC that the coalition forces had used different Pakistani airbases after the 9/11 attacks and they are still using some bases in emergency situations. Interestingly, however, on July 18, 2006, Tariq Wasim Ghazi told the PAC: “The United States has withdrawn its forces from the Pakistani airbases it was using during the war against the Taliban in Afghanistan.” As stated by the Defence Secretary six months ago: “None of the Pakistani airbases is being used by the allied forces, as had been the case in the past.” Ghazi’s comments came in response to queries by parliament from the Muttahida Majlis-e-Amal (MMA) who asked him if the US army was still using Pakistani airbases.
http://www.thepost.com.pk/Arc_OpinionNews.aspx?dtlid=81110&catid=11&date=02/05/2007&fcatid=14