Excellent photographs here.
My first reaction: It's not right to party at Ground Zero, where so many people died. You don't dance on their grave.
Second reaction: Where are you going to go? It's New York City, and people want to go out on the street and celebrate. Even if most people regard Ground Zero as a solemn, sacred place, some people are going to think it's... the place.
Third reaction: Even though dancing on the mass grave of the victims is a nontraditional way to celebrate the death of their murderer, it makes sense. If we believe or imagine that the dead are spiritually or symbolically there and that they could know what has happened and respond, then this is the place of celebration, and it's fine to congregate with them for the revelry.