When the [Westminster] Confession was written as well as now, there were men who were accustomed to asseverate that to affirm that God had freely and unchangeably ordained whatsoever comes to pass were to declare God the sole cause operative in the universe, to destroy the freedom of the human will, and indeed the reality of all second causes, ... Accordingly, the Confession adds the caveat, that God's foreordination does not make him the author of sin, nor offer violence to the will of the creature, nor take away (but rather establish) the liberty of contingency of second causes. In other words, the Confession guards it readers against being misled into supposing that the divine government of the universe according to an eternal plan excludes the administration of that government through instruments; and protects the reality and real efficiency of all second causes, free and necessary alike, while affirming the reality and real efficiency of the first cause as the determiner of the course of events in accordance with the primal plan. [B.B. Warfield, "The Confessional Doctrine of the Decree," in Selected Shorter Writings (ed. by John E. Meeter; Phillipsburg, NJ: P & R, 1970), 1:98-99]
According to Warfield, the rejection by the Westminster Confession of God being the "Author of Sin" is meant to reject the view that God is the "sole cause operative in the universe." Also, the insertion of the predicate adjective "reality" to the nominative phrase "second causes" shows that those second causes are to be regarded as actually and truly operative, not just an appearance or occasion for the actual cause, hiding behind the "second cause," to work. The first cause, God, is indeed the determiner of the course of events, but it is determined without the rejection of the real legitimacy of free and necessary creaturely causes.
Any position therefore that claims God as the Author of Sin in the rejection of the true ontological reality of second causes is contrary to the Westminster Confession and contrary to the Reformed faith, regardless of how one wishes to define the word "author." To claim God as the "Author of Sin" in the conceptual form (regardless of terminology used) as articulated and rejected by the Westminster Standards, is to reject the Reformed and catholic view concerning the nature and the decree of God.