You misunderstand.
It is the Pakistani possession of Nucs that ensures that whilst the US would be desperate to do something if they got a warship damaged/destroyed, despite the loss of face would do nothing. This would be a salutary lesson to all small nations of the advantages of getting some big bang capability.
The US (and others, to be fair) is very willing to throw it's weight around when the balance of power is extremely one-sided.
I notice, however that the US loses it's balls once the other side has some buckets of sunshine. For example see North Korea vs Iraq or Afghanistan or etc etc.