But ... it's the removal of artificial, man-made life support. It's not like someone is being given a syringe of cyanide. What about the Jehovah's Witnesses (I think it's them) who refuse blood transfusions or any kind of medical intervention if they're sick? Are they sinning if they refuse to ingest pills or take needles or have surgery?
I don't see how an ommission can be the same as an action.