Especially since screen tapping could be registering a "refocus to this point" action. Also since camera phones have no viewfinder, I use the screen to compose my shot. So to avoid obscuring what I am going to take a picture of, I keep my hand clear of the screen most of the time. That makes the act of triggering the shutter a more visible and deliberate act than just pushing a button that my finger is already touching. These probably don't matter to most people but are fairly frustrating to me. (the fact that I almost always shoot landscape, not portrait, may be the cause of the discomfort for me, since it changes my hold on my phone to something more like the hold on a camera.)