It is a technicality of treaty law.
The US recognizes the Vatican as a sovereign nation.
the Vatican has long maintained that its priests are first and foremost liable to Vatican law.
Personally I believe that those priests who commit such acts should be dealt with by the nation in which the crime occurred.
So is that why Cardinal Law is living in a palatial apartment rather than in a prison in the US?
This was one of the reasons for the Reformation in England. People were sick and tired of Catholic priests breaking the law and claiming immunity from secular courts.