He had no legitimate reason in the world to believe that he would be allowed entrance to Charleston harbor with a fleet of warships. That would be like you coming to my front door at 2 AM with a shotgun over your shoulder demanding "peaceful" entrance and, when I denied you as any sane person would, you started threatening to kick the door in on your own and fight your way through me with your gun.
The good people of Charleston percieved Lincoln's fleet as a very real and very hostile threat to their safety and well being so they preempted it before it had a chance to sting them. How you fail to see this is baffling considering that this exact same strategy was used to remove Saddam from power for the exact same reasons only a few months ago.
So what is wrong with my expecting access to my property, regardless of the hour? Sumter was the property of the U.S. government. The Davis regime was attempting to starve it into submission. Lincoln did what any reasonable person would do, attempt to peacefully and openly deliver food to the fort. There was no threat to anyone in Charleston at any time.