While all (most? many?) extraordinary renditions are kidnappings, not all kidnappings are renditions. The difference is that when doing extraordinary renditions, we are delegating the incarcaration/torture/interrogation of the victim to _some other actor_ specifically so that we can technically not be performing horrible acts. That's what makes it a special kind of kidnapping.
Kidnapping is only one of the ways we ingest people to treat in this manner, if I recall right. Presumably one could be arrested and then renditioned elsewhere -- the rendition is the part where we send someone elsewhere to be harmed in ways that our laws don't permit us to do ourselves.
Kidnapping people to face punishment without fair trial seems wrong no matter who does it.