The light emanating from the screen of the phone is generally bright enough to distract one from watching the screen and draw the eye towards the new light source. This detracts from the enjoyment of some people. In some areas it is considered extremely rude. In Japan and Korea they have cell phone blockers installed in movie theaters to stop such activity. Think about it... to some people (myself included,) it is the ultimate in rude, self centered behavior.
This is a sad comment on community events in general, when there is much less of a sense of, well, community in the community, and more people have chips on their shoulders. If someone won’t be polite and considerate in a movie theater, they likely won’t be in the library, in the grocery, in the post office, while driving downtown, etc. Sometimes I wonder if zero tolerance policies, however, only move the chips to the other shoulder.