Usually, if you're in church, you shut down the cell phone out of respect for God and the people around you. If you're the President, enjoying mass on the momentous occasion of your second inauguration, one would assume you'd be so overwhelmed with honor and reverence that you wouldn't want to waste time tapping away at your screen. Even if you don't really care about the whole "church thing" and you'd rather be goofing around on Twitter, you'd probably be aware that it's crass and unbecoming. So, you'd wait until service was over.
