Many determinists and open theists object to free will coexisting with exhaustive foreknowledge on the basis that God's foreknowledge of events implies that He has predetermined the event. They say that God can't know a future event in an undetermined world, but could only know probabilities of future events. They say that God either knows the future because He determined it, or because it is determined by the physical state of affairs as they currently are.
I've never thought that either of these were compelling. I've really never seen a determinist or open theist
argue for either point. They seem to simply assume it. Why can't God know future things simply as part of Him being omniscient? If omniscience is the knowledge of all true proposition, then it seems inherent in the definition that God knows future events without needing to deduce them from current physical states of affairs or personally determining them Himself. Not only that, but that definition seems to include counterfactual states of affairs as well; what things would be like if certain things are different. But it seems to me that, at least on the second option (deducing from current states), God cannot know these propositions, since they don't obtain in this particular world.
William Lane Craig has pointed out that this objection rests on the assumption that a certain form of truth-maker theory is correct. He argues that the person offering this objection can't simply assume something that controversial without arguing for it. I haven't seen an argument to make me think that these CCF's absolutely need to be grounded in the way the objector wants them to be. As Plantinga says, "It seems to me much clearer that some counterfactuals of freedom are at least possibly true than that the truth of propositions must, in general, be grounded in this way."
However, the determinist and open theist also seems to be unaware of the good attempts by libertarian free willers to give a ground to this knowledge. Alfred Freddoso has given such an argument. As Freddoso says, "it seems reasonable to claim that there are now adequate metaphysical grounds for the truth of conditional future contingent Ft(P) on H just in case there would be adequate metaphysical grounds at t for the truth of the present-tense proposition p on the conditions that H should obtain at t."1 In other words, the metaphysical ground for the truth of a counerfactual proposition is not that the event actually exists, but that it would exist if the certain state of affairs were the case. This parallels the grounding of the truth of future propositions. It is either true or false that my Chiefs will win on Sunday. The truth depends on what actually will occur at that time. That is how it is grounded.
Even if this account fails, the objector still needs to give us a reason for why God's knowledge needs to be grounded like this before it is to succeed.
For more on this, see here and here and here and here.
1 Freddoso, Alfred, On divine foreknowledge: (part IV of the Concordia). Cornell Univ Pr, 2004. Print. 72.