Because any leisure is a personal choice which you can choose to do or not if you really are worried that you can't get in contact with your nanny then don't go

Because any leisure is a personal choice which you can choose to do or not if you really are worried that you can't get in contact with your nanny then don't go to the concert where they require you to not bring it, don't go to the movies, don't go anywhere you can loose connection and don't go flying. It's perfectly fine if you don't want to take that chance but it's hardly an argument for why someone shouldn't do a show where they ban cellphones.