The government of the Church overrides that of the State- especially in regards to priests because an offense by a priest is so much more serious than that of a layman. The question might be raised as to whether the Church will prosecute offenders properly. Well, are we sure the US does a good job?
How many government officials are guilty of this same type of crime? Can we be sure that the offending priests would not have been protected by the same system of corruption here in the States?
The argument is over who has the duty to prosecute clerical criminals. Remember, if the US is given that right, then all other nations should have equal rights to prosecute/protect according to their laws. That would imply that all other governments have more superiority than the Church. Whatever a person may think about the Church, this seems like a logical stance. If the Church has truly been established BY GOD to "teach and govern" then it should stand up for this right/duty.
Also, take into account that Catholics know that God's punishment will be far worse than anything that this world can mete out. Something that victims have forgotten in their rush to "punish" those who have hurt them. Jesus said that having a millstone around the neck and being thrown into the sea would be no comparison to the punishment.
BTW- the Catholic Church has traditionally held that lethal sentences may be given. It has softened this stand recently on the argument that the prison systems are sufficiently secure to contain dangerous criminals. (And perhaps the realization that the judgements of man are too often incorrect.)