Its out of their jurisdiction and is a bad precedent. Police are for municipalities. It's why we have different levels of law enforcement, ie city, county, state, federal. Fuck Bloomberg and his overreaching liberal self.
As long as they are just watching, then they are not exercising any police rights. Let NY pay for extra work. If it helps catch a terrorist, then great. If not, then NY has paid for extra work and not the feds.
It's like saying that one city does not have the right to go to another city and ask questions. Should they inform the city? Probably. But if they suspect terrorism, should they watch and confirm, or say something and take the chance that loose lips sink ships, all in the name of turfs?