Of course, this is exactly what Paul's objector says in Romans 9 when dealing with the fact that it's God who has mercy and God who hardens hearts. I've also made similar remarks when "defining free."
I'd like to add one more thought.
Within the Bible itself, there are cases where unthinking things (animals and a tree) are held responsible for their actions. By held responsible, I mean they were punished for what they did (or did not do). This clearly refutes the assumption that one must have free will in order to be held responsible.
Case 1: Genesis 9:1-6
1And God blessed Noah and his sons and said to them, "Be fruitful and multiply and fill the earth. 2The fear of you and the dread of you shall be upon every beast of the earth and upon every bird of the heavens, upon everything that creeps on the ground and all the fish of the sea. Into your hand they are delivered. 3Every moving thing that lives shall be food for you. And as I gave you the green plants, I give you everything. 4But you shall not eat flesh with its life, that is, its blood. 5And for your lifeblood I will require a reckoning: from every beast I will require it and from man. From his fellow man I will require a reckoning for the life of man.
6"Whoever sheds the blood of man,
by man shall his blood be shed,
for God made man in his own image.
After rescuing Noah, God reaffirms His faithfulness and now extends all living things as food for Noah when previously it was just green plants(v.3), warning against eating flesh with blood. And then v.5 is my focus - "And for your lifeblood I will require a reckoning: from every beast I will require it and from man. From his fellow man I will require a reckoning for the life of man."
Yet, in spite of the obvious lack of "free will." God holds them responsible.
Case 2: Exodus 19:12-13
12And you shall set limits for the people all around, saying, 'Take care not to go up into the mountain or touch the edge of it. Whoever touches the mountain shall be put to death. 13No hand shall touch him, but he shall be stoned or shot;[a] whether beast or man, he shall not live.' When the trumpet sounds a long blast, they shall come up to the mountain."Moses is up on the mountain where God is giving the Law, and He gives this law as well to Moses, warning the people of Israel to not even touch the mountain. He then gives the punishment for touching such a mountain, death by stoning or arrows.
And then we get to the clincher. Not only is this a command for thinking rational beings like people, but it's one for unthinking beasts as well.
v13No hand shall touch him, but he shall be stoned or shot;[a] whether beast or man, he shall not live.'Once again, unthinking beasts are held responsible. No trace of free will needed.
Case 3:Mark 11:12-14;20-21
12On the following day, when they came from Bethany, he was hungry. 13And seeing in the distance a fig tree in leaf, he went to see if he could find anything on it. When he came to it, he found nothing but leaves, for it was not the season for figs. 14And he said to it,(irrelevant parts to my point snipped)
"May no one ever eat fruit from you again."And his disciples heard it.
20As they passed by in the morning, they saw the fig tree withered away to its roots. 21And Peter remembered and said to him, "Rabbi, look! The fig tree that you cursed has withered."
Does the tree have free will? It wasn't even the season for figs. Yet Jesus, righteous and just by definition, perfectly obedient to His Father's wishes, righteously and justly curses the tree for failing to meet God's demands. No free will required.
So the assumption "moral responsibility presupposes free will" fails biblically. Not only is it never stated, but it's clearly refuted. We're held responsible because God chooses to hold us responsible. May He have mercy on our souls, as there's no where else for us to turn.