Even though many of us Arminian e-pologists (as we are affectionately known) have dealt with this issue (see here, and here, and here, and here, and here, and here, and here, and here, and here, and here, and here), it is still thought to be just the death blow to our argument.
Here's how it goes. We say, "determinism means God is the author of sin, because everything that happens happens because God has made it happen." The Calvinist says, "well so does Arminianism because God knows about sin and lets it happen anyway!"
Okay, imagine the worst sin you can imagine. The worst evil that could ever happen. Now, imagine that a guy named Evil Evanston is performing this heinous act. If Evil Evanston has free will, in that he is not under constraint by a force outside his own will to act, and has it within his power to not act as he will (the power to do otherwise) then it is Evil Evanston who is choosing for himself to sin and then carrying out that sin. He authors the evil. If God, on the other hand, did not grant us free will, but has deterministically ordered the universe such that everything that happens HAS TO happen, happens necessarily, then God is the origin of all events and choices. God is the one who came up with the evil thought and evil act that Evil Evanston is going to commit, and nothing but what God has determined to happen could otherwise occur. Evil Evanston is just carrying out the pre-scripted programming that God set up. God, in this case, authors the evil. The author of the evil is the morally responsible party.
The Arminian knows, as the Bible teaches, that God only does good. All good things are from God. In fact, since God is by nature good, it is logically incoherent to think He could make evil. We measure acts against God's nature to determine if they are evil. If all acts are from God, then no act is evil. But we all know there are evil acts.
"But," the Calvinist will interject here, "God allows evil to occur on the Arminian view! He just stands there and watches Evil Evanston commit his heinous act, which is obviously in the same ballpark as determining it will happen!"
I'm not sure if Calvinists have had their coffee when they say something like this. Allowing a sin is tantamount to making the sin happen? That's just bad logic, as I pointed out in my response to James Swan. Say I knew what Evil Evanston was going to do, and I had the ability to stop him. Would I, as Swan says, "share responsibility" for the sin that Evil Evanston commits? No. There would be two separate actions taking place; Evil Evanston's sin and my allowing his sin.
Is allowing someone to sin a sin in itself? It depends on a whole host of things. Parents sometimes will allow their children to sin to allow them to experience the consequences of that sin. The police will sometimes arrange a situation where they know a criminal will sin so they can arrest the criminal. In other instances, I think it would be utterly negligent to allow sin. If someone was going to kill someone and you had the power to stop it. There are obvious instances where non-action would be negligence (which is a separate sin than the sin committed).
Okay, so the Calvinist must be saying that God is utterly negligent on Arminianism because He isn't stopping sins He knows about and has the power to stop. Is this the case? No. As creatures of God, He has charged us with certain moral duties, one of which is to help people when we can and not be negligent. That is a moral duty that flows from God's moral nature. But, as the dispenser of moral duties, God does not issue command to Himself. God has no duties to follow. He holds the power over life and death and has the right to take away the life of whoever He wants whenever He wants. Obviously, we don't share that right. In fact, that is one of the things that makes His grace and mercy so amazing. God had no obligation to rescue us from our sins, to conserve us in being, or to ever show anyone any grace. But He does!!
Okay, so can we say that something is still wrong with God allowing evil? Would an all good God do such a thing? If God has decided to give us free will, then there may be no way for Him to create a world where no evil happens. Free creatures may always choose evil in no matter what set of life circumstances they live in. Could God stop them? Well, if He has decreed to make them free, no. It is logically incoherent to speak of making someone freely do something. It is as nonsensical as speaking of a married bachelor or square circle. Beyond this, it is probable that God has morally sufficient reasons to allow the evils that currently happen.
But, on deterministic Calvinism, you can't postulate that God possibly couldn't create a world where free creatures always did good, because if Calvinism is true, we don't live in a world of free creatures. God could have created a world where no evil happens if He is making everyone do what they do.
So, it is impossible for God to make sin. God allowing sin is far different than causing sin. God has no duty to stop all sin. He may not be able to create a world where we all freely do good. And He may have morally sufficient reasons to allow sin. But what could possibly be the benefit of causing all sin?