Sometimes my kids use to *borrow* cookies from the cookie jar. Once in a while I would catch one of them. Punishment followed. When asked, "So and so did it, why aren't they getting punished?".
I would respond, "Does it make what you did right?"
Moderators are players. They volunteer to help moderate. If they are in the middle of a good fight and miss something, no one here will complain about it.
Also keep in mind they are not always around. Of course, if we did not need moderators, this would all be a moot point.