how so?
It's that an all-knowing God and free will are incompatible:
If God knows the future, then events unfold as God foresees
If events happen as God foresees, then they happen by necessity
If events happen by necessity, then we cannot choose otherwise
If we can't choose otherwise, then we have no free will
So, if God knows the future, then we have no free will
Unfree creatures are not morally responsible for their actions
It is unjust to punish an unfree creature for its actions
God punishes his creatures for their actions
So God is unjust