Because if the omniscient person knows it's going to happen, then it's going to happen. He's not forcing you to do it. He just knows that you cannot NOT do it - you cannot do otherwise and you won't do otherwise. Neither can't or won't is caused by him. We deduce this from knowing (or proposing in this instance) that he's omniscient. We don't have to know how he knows these things, what makes him certain, what other forces are involved, etc., to make this deduction.
Taking him out of the equation takes the unknown cause of his certainty out of the equation. Without that certainty, saying "if you can't do something, you won't," is still logically correct. Saying "if you won't, you can't," is not logically correct.
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