The first is that the social structure of a society is an internal, domestic matter and when a foreign government intervenes they are violating the principle of national sovereignty.
The second reason suggests that the reader visit the reports of RAWA Amnesty International, UN, Boston Globe and various other news media and humanitarian groups; these have shown how much worse off Afghanistan is today than it was under Taliban.
The third reason is very simple and transparent. Anyone who knows anything about U.S.-Afghan history in recent years understands the real reason for the invasion was the Taliban's refusal to grant the U.S. right of way to build their pipeline to Caspian petroleum.
Ahmed Karzai, Drug Runner
It is well known that Ahmed Karzai, the brother of the U.S.-installed puppet, Hamid Karzai, is the narco lord of the country. Ahmed is operating a drug wal-mart in Afghanistan supplying to the Middle East, Europe, and South Asia in plain view of the U.S. military, the CIA and Mossad. It's interesting that neither NATO nor US troops are able to stop this drug trade, when Taliban eliminated the majority of the drug trade during their rule. The CIA's use of drug money to fund their operations against unfriendly governments in Latin America is another well-known "secret". In Afghanistan, we posit that an organised Mafia, comprised of the Karzai brothers, the CIA and Mossad, is working to control - not eliminate the multi-billion dollar heroine traffic out of Afghanistan. 
Without active support of Pentagon and CIA it is not possible to export drugs prepared with more than 8,000 metric tons of opium. To ensure undisturbed trade, the U.S. appointed all Northern Alliance drug lords at key posts in Afghanistan and the most prominent appointment was none other than President Hamid Karzai. Karzai's brother, head of Kandahar's provincial council is a proven drug trafficker facilitating the transportation of heroin from Kandahar eastward through Helmand and out across the Iranian border.
The US has now focused its attention on Pakistan for a couple of reasons.
The first of course, is destabilization which they think will lead to their control.
The second is to mask their obvious failure in Afghanistan. Establishing their military in Pakistan would give them a base for continued operations in Afghanistan after they have been forced to leave. When they do leave, instead of admitting defeat, they will brag about their defeat of the Taliban in Pakistan and the generals will blame politicians for not allowing them to kill enough people.
Vietnam anyone? The Kerry-Lugar Bill The latest misadventure of the U.S. was the recently approved Kerry-Lugar Bill. This bill is basically asking Pakistan to surrender its sovereignty to the US for just $1.5 billion per year. The U.S. thought that the Pakistani nation will sleep and not respond. It's another gross miscalculation on the part of the U.S. They may have some Pakistani politicians in their pocket, but the people will never surrender their freedom to this foreign invader. Short of surrender, the U.S. hopes to Palestinize Pakistan with perpetual war. This bill asks the Pakistani government to take action against any target within Pakistan that the U.S. tells them is a "terrorist hideout". It happened this week when the Pakistan army attacked a suspected "hideout" killing "dozens of people" (CNN Europe).
The Kerry-Lugar bill is designed to legitimize their future operations. They did the same thing by passing the Patriot Bill in March, 2006. In that case, they plan to control any possible domestic reaction to their foreign and domestic policy and failing economy with legislation enshrined for that purpose. They are doing the same thing in Pakistan.
For example, if the US tells Pakistan to strike at Quetta, or Mureedkay (both places mentioned in the bill) then, according to the bill, Pakistan army must do so. Further, the bill says that the US authorities can question any Pakistani official working on Pakistani nuclear program if the US thinks that person is involved in "suspicious activities".
The farce continues... the US demands that Pakistan must get a certificate of good behavior from India (Pakistan's arch enemy), showing that Pakistan is behaving well with India before the US will give aid to Pakistan.