You are right, NATO should not be there when the people of Afghanistan do not want them. But the NATO countries were given the mandate to be in Afghanistan to bring peace to the war-ravaged country which they failed to achieve in the past eight years despite all the resources at their hands.
Not only that the NATO and US forces failed to bring peace to Afghanistan, they imposed a puppet government of Hamid Karzai and a lot of corrupt administration. Majority of the ministers in Karzai administration are from one group or another involved in the pre-Taliban civil war in that country. Further, the administration is corrupt and the ministers and senior officials are involved or having links with the smugglers and other gangs.
In such a situation, the ultimate suffering lot is the Afghan people, who are being sandwiched between the corrupt Karzai administration, the foreign troops who pay little heed to the voice of common Afghans and even the Afghan government and the resurgent Taliban. A lengthy discussion, that will need much time and space to conclude -- best wishes.