ISI is the government inside the government in Pakistan. What ISI dictates is what translates into action on the ground. All that hullabulloo about democracy in Pakistan is pure baloney. ISI has for its Godparents, the army, whom nobody, repeat NOBODY can touch and remain relevant (read alive) in Pakistan. The all powerful army wants a piece of the Afghan pie without seeming overtly greedy about it. On the other hand, it wants India out of the Aghan scene. The only way it can proceed forward is to take the help of the Taliban.
The taliban keeps India occupied by its constant forays into Kashmir, Kerala and other parts of the country, fomenting trouble and mayhem. That gives Pakistan enough time and resources to concentrate on its north-western border. Hazardous as they are, they can be relied to safely transmit messages, people, resources and strategy across the border without unduly being worried about either surveillance or complicity.
Strike at the ISI and Taliban would automatically fall. But the US, which has the Pak government by its balls wont do it, simply because a frail Pak is of much nuisance value vis-a-vis India’s rise as a world power. 26/11 has, by all counts, made US into a paranoid state which is pumping untold millions into Homeland Security. Where are these millions coming from? From arms sale for one. Selling it to India (one of the richest emerging markets for arms) is possible only if India sees the purchases as necessary for its security against Pak. And therefore US will continue the dalliance with ISI till another (God forbid) another 26/11 happens.