Fear.
The Taliban have shown time and again that they will not hesitate to use violence against those who do not follow their ideology.
I've read and it was reiterated by an Afghan doctor who spoke at UH, that one of the reasons people in Afghan and Pakistan follow the Taliban is because they actually represent a stabilizing force. Particularly, the courts and the government were so corrupt in the past, that any kind of justice was impossible. Sharia provides something Afghan society that they haven't had before. So, that is one of the reasons that the men join, that and the Taliban represent a group that is fighting an aggressive force that has killed a lot of innocent people. Therefore, revenge is also a huge factor.
I really wonder how many people actually believe in the Taliban's version of Islam? My guess is that it's probably similar to any other extreme religious group. Therefore, as soon as the outside forces quit destabilizing the society, quit providing weapons and money to the crazies, Afghan society will start to sort it's way out.