The fact that it's in the immediate interest of public safety. Watch the video from TFA; it looks like the event was far larger than anticipated, with completely inadequate crowd control. People were being shoved by the crowd through doors and down stairs. Mobs of people like this can easily knock someone down and trample them to death; it happens when there are fires in crowded space, or even when people are excited about being let into Wal-Mart on Black Friday. As the event had been announced through twitter, and the vast majority of the crowd was teenage girls with cell phones, so the hope was probably that getting a message from the official Twitter account itself would help disperse the crowd a lot better than the single cop getting up there with the megaphone, causing the crowd to just get angry.
When there's an immediate threat to life and health, compelling someone to make an announcement to disperse the crowd is an entirely reasonable thing to do. This is essentially the same case as that of calling "fire" in a crowded theater; inducing a panic in a confined space can cost lives, and likewise refusing to cooperate in trying to disperse a mob can cost lives as well.