Alexander Rusev was never, to my knowledge, actually portrayed as Russian (not least because he has the Bulgarian flag tattooed on his right shoulder); he was presented as Russia's 'champion'. Besides, in many sports there is a history of athletes switching national allegiances for personal gain.
Further to this, just how many foreign wrestlers are portraying characters from countries other than theirs? Fritz Von Erich was a German heel player by a Texan; the Iron Shiek (Iran) briefly played Col. Mustafa (Iraq); the Samoan Rodney Anoi'a played the Japanese sumo Yokozuna; and most of the Russians don't actually have any Russian blood, to name a few. It's not a new thing, and, as others have noted, what's the difference between, say, Robert Carlyle and Robbie Coltrane playing Russians - badly, I might add - in James Bond films and non-Russians playing Russian characters in wrestling?