It's not quite as cut and dry. If it were any other nation, we'd sanction them like Iran or Syria. But ISS knows that we fear an isolated and further radicalized Pakistan (that's the kind of respect that Nukes buy you) gives them some leeway. So long as the evidence isn't particularly overt, we'd be reluctant to press hard on them.
Of course, India knows how important Pakistan's stability is to us right now. They likely see this as an opportunity to get additional concessions (i.e. trade agreements, military assistance) from the U.S.