It's not a matter of logic; it is what the system teaches. It teaches that God is not the Author of Sin, and it tells you how that is not so.
Their Westminster Confession says:
God, from all eternity, did, by the most wise and holy counsel of his own will, freely, and unchangeably ordain whatsoever comes to pass: yet so, as thereby neither is God the author of sin, nor is violence offered to the will of the creatures; nor is the liberty or contingency of second causes taken away, but rather established.
Now, I can use whatever logic I want, but if it ignores the statements of their own system, then it is faulty to charge them with a fault that is not the result of the system they have actually established.
I understand that in Canadian Football there are 12 men on the field. Therefore, American Football doesn't have enough men on the field. (I can deduce all kinds of things from that about American Football, but none of it would be valid. American football must be seen in light of its own system.)
In Calvinism, what is the difference between the eternally damned and the eternally not?