The problem is that the Taliban are a terribly large group. It's de facto an army, not just a small group of crazy terrorists (even though those crazy minds are a part of it). These men somehow must be included in a future democratic society.
They know that they can't win a war in Afghanistan. They don't view the negotiations just as a win, because they don't really have an alternative.
When you want to have a peace in Afghanistan, what else can you do? Among the Taliban there are men who haven't known anything but war in the last 34 years...