The answer, we think, is that it is indeed sometimes within our power to determine what God believes. We do not thereby cause any changes in God, nor limit His omniscience, for it is neither change nor limitation in God that some of His states count as beliefs of what we do in virtue of our doing those very things. 'A thing will not happen in the future because God knows it will happen, but because it is going to happen, therefore it is known by God before it does happen.' as Origen said. This is, perhaps, what God's foreknowledge is all about.Widerker, David. Religious Studies, Vol. 23, No. 1 (Mar., 1987), pp. 19-28 (emphasis mine).
So, if God determines some human beliefs, that makes those humans into puppets. Man must be libertarianly free, and so woe to those who would dare take man down from his exulted place and try and remove his powers of creating events ex nihilo. Libertatrian freedom is so valuable that some would make God into the puppet, just so long as they get to keep their powers of libertarian freedom. However, if God remains "unpuppeted" in this scenario, then Arminians are compatibilists about God freedom in some cases. Thus, they wouldn't believe compatibilism necessarily rules out freedom, losing the principle objection to Calvinism.