It's common sense.
A lie about a small matter is not the same thing as publicly slandering a man or attempting to destroy his reputation. For example, coming forward and saying that a Supreme Court nominee tried to sexually assault you 36 years ago, when such isn't the case (or you can't remember who sexually assaulted you 36 years ago because you were drunk).
And sin is always to be condemned, even if we ourselves sin. Obviously it behooves us to cut our neighbor some slack, especially regarding venial sins which virtually everyone commits. But it would be foolish to let murderers, rapists, traitors, and other serious criminals off the hook, because "we are sinners ourselves!" or "who can cast the first stone?" (perhaps yesterday you got angry while in traffic).
God is MORE just than man, not less just. So if we (being evil and imperfect) can distinguish the difference between stealing a candy bar from a store, and embezzling a widow out of her life savings, don't you think God makes such a distinction as well?