The fact that it might ruffle a few feathers diplomatically is hardly something the CIA considers when they handle operations like the one they're describing. Also, if they did grab him, trust me, he wouldn't be heard from again. Considering Snowden has an army of governments after him, it would be easy for the CIA to run a disinformation campaign to keep people guessing who really kidnapped him.
It would be easy for the CIA to rotate him between several of their "black sites" around the world and basically let him wither and die in one these miserable places.