Ethics is not enough

Freewill theists raise two major objections to Calvinism: exegetical objections and moral objections. However, the moral objection cuts both ways.

Moral realism is not enough. You must live in the kind of world where it's possible to do the right thing. You must live in the kind of world where you're not confronted by moral dilemmas. Objective moral norms are not enough if, through no fault of your own, you find yourself in situations where you can't apply moral norms consistently. If there's a mismatch between morality and reality, where morality and reality represent divergent paths, then moral realism becomes a kind of fatalistic, Kafkesque entrapment, where you are doomed to do wrong no matter which way your turn. 

In varieties of freewill theism, God lacks sufficient control of the variables to ensure that our duties and our circumstances align. In the case of open theism, God doesn't even know where the future is headed. In the case of Molinism and classical Arminianism, the libertarian freedom of human agents limits God's options. God can only work with what's left over. Free agents may take the best options off the table. God is out of luck. 

Indeed, there are freewill theists who admit that libertarian freedom leads to moral dilemmas, because God can't coordinate all the variables to guarantee a licit choice in every situation. Yet it would be unjust for God to punish agents for wrongdoing if, through no fault of their own, no licit course of action was open to them. So God must suspend morality under those circumstances. A zone where good and evil don't apply. 

In the face of moral dilemmas, some ethicists propose threshold deontology: dire situations where moral norms are suspended. And that's the kind of train-wreck that freewill theism invites. 

John Frame has often argued that in ethics, the duties and situations must coincide (normative and situational perspectives). If you subscribe to absolute predestination and meticulous providence, then God can prearrange events to elude moral catastrophes. But in freewill theism, that's inescapable.