Secondly, these events have security, crowd control, management, etc, all with their own phones. Emergency services doesn't need a thousand simultaneous calls. In a genuine mass emergency there is a lot more important needs for network access than providing people the opportunity Instagram/Facebook about their experience.
If friends and family have to wait a little while longer to hear from you in an extremely rare emergency, that's a small price to pay for the large benefit. You can't fully experience a concert or show when you are distracted by your or someone else phones.
This technology makes the "no phones" policy work. I don't have to be made to feel like an a-hole calling for an usher to deal with phone users, or worry about them or their friends harassing or assaulting me for doing it. It makes the experience better for everyone.
A few weeks ago Joe Rogan spoke about how he's now using Yondr at all of his shows, because despite how expensive it is for him, it has significantly improved the show dynamics for his entire audience.