it is a disgrace, it is typical of the disrespect that people show for music,it displays an attitude that they cant be very interested in the music, these people would rather play with their latest toy,ok that is fine but dont go to a concert if you want to play with yourself.
if i took off my clothes or stared picking my nose or doing silly walks, it would not be acceptable, it is not acceptable for people to play with their mobile phones during a concert, it spoils other peoples pleasure and illustrates a selfish inconsiderate attitude