Probably because they think that the USA have greater moral principles (...)
Again you're constructing your argumentation on the false assumption that the death penalty is not "moral" under clearly defined conditions (i.e. the Rule of Law in liberal democracies, as opposed to the rule of power and arbitrary of totalitarian regimes), which is precisely what I have been disputing so far. Therefore, the rest of your answer doesn't hold water. Sorry.
You should be concerned precisely by China because the regime has little (if any) moral principles. Unless of course, you happen to have a very twisted conception of morality.
(...) death penalty sometimes results in killing the wrong guy (...)
I see. Throwing the baby with the bath's water again.
Let's make it clear: you're making a point against the infallibility of the prosecution process here, but in any case not against the legitimacy of the death sentence itself.
Finally, I leave it to Damian to catch up on your answer to his question, if he can make any sense out of it, as I'm not sure I understand what you mean actually.