A good question, assuming the premise is true. I think a lot of people doublespeak on this point: They claim the North was bound by the Constitution, however Northern abrogation of the Constitution was obvious at the time of secession.
The Confederate leaders should have expected a fight, but must've thought a demonstration of will would be adequate.
I would dispute the "abrogation of the Constitution", but the point of my post is how ridiculous the southron insistence on comparing their actions with the Founders is. The Founders did not secede, the launched a rebellion expecting to have to fight for their independence. There was nothing legal about their actions. The southron contingent, on the other hand, would have us believe that there was no difference between their actions and actions of four score and 5 years prior, which must mean that somewhere in British law there is a legal right to rebellion, and seem absolutely shocked that the Lincoln administration might object to their position. So you're damned right that they should have expected to fight. The fact that they did expect war and were, in fact, preparing for it is borne out by their creation of a huge army and their belligerent positon on Sumter and Pickens. And there is the fact that Davis did need the other 8 slave states in his confederacy if it was to have any chance at all to prosper outside the shadow of the U.S. He knew that those states wouldn't join without a war, and banked that he could win the war with them in the southern fold. As it was he only got half the states he wanted, and lost the war that resulted from his actions.