The US satellite was in a decaying orbit and coming back down to earth sooner rather than later.
Due to its decaying orbit the destruction of it will not result in a deadly, permanent debris cloud in a useful satellite orbit position like China's did.
The missile used to destroy it is an anti-missile missile and would be useless against satellites that are not in a decayed orbit.
http://en.wikipedia.org/wiki/RIM-161_Standard_missile_3
Everyone knew that they could before and like I said above, this missile isn't capable of destroying a fully functional satellite.
That would have been an act of war.