They are typically a little wary around humans and move away rather rapidly once you get within even 50 feet in an open area. It always strikes me as odd that such a bold bird hunting insects - something we may inadvertently flush up more of while walking nearby - is always so concerned about us. A lot of other invertebrate feeders vacillate from guarded to downright tame depending on the situation. This can occur when they are tired or very hungry, or when one certain bird simply does not care about your presence for whatever other reason. But those Eastern Kingbirds always seem to care.
Take a look at the photo again and maybe you'll see in particular from this angle what I saw...or rather, did not see. I noticed how disguised those eyes really are as I could not be sure when this cautious bird was looking at me or not. Plenty of other insect-eating birds have bright or highlighted eyes, from the Blue-gray Gnatcatcher to the Nashville Warbler and many more. This species never wants you to know where it's looking as it sits in the open and exposed to the world, but they will go after perceived threats with limitless vigor and capture prey before the victim even knows the bird is looking at it.
Photo by Scott Kruitbosch © Connecticut Audubon Society and not to be reproduced without explicit CAS permission