"Their own methods" related as to being covert...
I simply can't see what good a big army would do there, who would they fight?
What must be understood is that there's no coastline or anything to push the target back too, they can withdraw forever, crossing into countries like Tajikistan (1,206 km border), Turkmenistan (744 km border) or Uzbekistan (137 km border).
Don't forget, Bin Laden and his party will know of every big move by the army, since it'll be closely monitored by the media.