I don't get why American forces are in Afghanistan?

Are they are after Bin Laden, considering he is probably not hiding there--what idiot would? If they are not after him, are they there to defeat Taliban, which is inherently supported by majority of Afghanistanis--despite what the US media tells the public? Also if US decides to get careless and lowers rules of engagement and mistakenly kills a large amount of civilians, how do you think the rest, 1.7 billion Muslims and the fastest growing populace, will react? Is it not better to get the **** out of there and concentrate on fixing domestic problems?