Part checks and balances, part good ol’ fashioned egalitarianism. From page 11 of the opinion:
It is clear … that the “constitutional freedom from arrest does not exempt Members of Congress from the operation of the ordinary criminal laws … Indeed, implicit in the narrow scope of the privilege of freedom from arrest is, as [Thomas] Jefferson noted, the judgment that legislators ought not to stand above the law they create but ought generally to be bound by it as are ordinary persons.”
Between the warrantless wiretaps and the keeping secrets from Congress, I don’t know how much gloating we want to do over this one.
The opinion’s here. Page 16 is pretty much all you need.