Of course it is their market-given right not to spend its money but I am 100% sure that the US economy cannot prosper if its best companies hoard cash on this scale and I am as sure that competitor regions are happy to see them idle that cash away.
To be clear, I am not suggesting that any fund breaking the buck will cause a destabilizing run on other moneymarket funds it is possible that an individual fund could have a credit event that is specific to it and not trigger a broad run only that policymakers should recognize that the risk of a destabilizing run remains.