Even now Radical Islamic groups, strongly represented in the Pakistani military and tacitly supported by the government, are destabilizing the country and the region and providing critical support to the Taliban regime in neighboring Afghanistan.
Many members of the congress strongly feels that the war against terrorism cannot be over unless until Pakistan is brought to justice.
Therefore one day America is going to bomb Pakistan anyway. If that's what going to happen then it doesn't matter if India attacks Pakistan or not.
As we know Pakistan is not capable of winning conventional war with India. We would be happy to take out Pak nukes and then leave Pak at the mercy of India so that we don't have to spend resources in order to bring Pakistan to justice.