Celebrities are on busy schedules, its not a habit for Katy and im sure they let the main people in charge know she would be late its usually just the ones in the crowd who boo
Who has the guts to boo someone in real life, it just shows how tacky some people are.
It's no different than attacking people online really. Neither are good but they happen. Usually booing is more organic and is the result of that persons actions or inactions.