I share your concern but the explanation may be simple. The Taliban, the "fiercest" fighting force" since the worlds 8th largest army, Iraq, is running, surrendering and doing every thing but fight. The likely situation is that we have been fed a load of propaganda about the 10 ft tall Taliban. We shall see.
We should just let the Northern Alliance have Afghanistan--sounds like they've earned it and expect it. We are there to destroy the taliban. Once we've done that, we should leave it be and let the new regime know that they will suffer the same fate if they mess with us. I fear we're going to stick our nose into their affairs, set up one of our disasterous puppet governments, (which earns us no love and usually stabs us in the back anyway) and then be stuck dealing with the consequences.
So what if we leave the region unstable? If they're busy with in-fighting they'll have less time to be a threat to us. We are definitely giving the Northern Alliance the wrong signal (arming them and encouraging them to attack the taliban) if we don't want them to take control. The Northern Alliance is cheering us on, but I fear we won't be happy until we've turned their cheers into curses. I have a feeling our idiotic foreign policy is going to make us a new set of enemies after the taliban is gone.