It may help to recognize that the spiritual inability of unbelievers to repent (without God’s help) is self-imposed. They reject God’s offers of mercy and salvation, but not because of any physical inability. Rather, they have absolutely no desire to repent because they hate God’s law and they love their sin. Jesus says as much in John 3:19-20.
And so the analogy of someone with Down’s Syndrome being imprisoned because he cannot act normal is not fitting because such a person does desire to behave normally, but is physically unable to do so the opposite of the situation with unbelievers. They do have the physical ability to obey God’s law and even to repent, but they have absolutely no desire to do so because they hate God’s law and love their sin.
A better analogy is a violent murderer who has been repeatedly offered forgiveness if only he will repent and stop murdering. But he doesn’t repent because he hates people and just loves murdering them. Is the man able to stop murdering? In one sense, yes, because his choice to murder is free and not due to external compulsion. But in another sense, no, because he will do what he desires, what he freely chooses: to murder. His spiritual inability to repent is self-imposed because he loves doing what he does. The conviction and capital punishment of such a wicked person would certainly be fair. Likewise, unbelievers do have a choice, and it is genuine. The problem is that they will choose to reject Christ every time because it is what they want unless God changes their heart. Unbelievers do not end up in hell because they have no choice, but rather because they do have a choice and, left to themselves, they will always choose hell (Psalm 14:2-3).