They ask “who am I to judge”. Right is it not for them to judge. God has already judged. To contradict Him is to make your own judgment. To submit to His pronouncement is nonjudgment.
It is impossible to perform the third spiritual work of mercy, which is to admonish the sinner, without exercising objective judgement.
“The basic reason that we admonish sinners is because their salvation may well be in jeopardy.”
http://www.catholicculture.org/culture/library/view.cfm?id=8158