Seriously? I wouldn't, but I don't think it terrible if people do, as long as they aren't noisy or flickering.
Who knows, might be their first time hearing an act, they are enjoying themselves and they want to share it with their friends whilst being in the moment. Such things really do happen, this is after all an age of instant communication. Ideas spread, and you cannot possibly know that a person isn't properly respecting the music just because they have their eyes on a screen. only that they aren't respecting it in the same way as you would.
That said, if the phone was glued to their ear, that isn't so good. People should go some place else when they get a call, and really the phone should be on vibrate or silent when people are performing.
But if it isn't disturbing anything (except your sensibilities), just leave people be. Let them enjoy things in their own way.