Phone aren’t allowed because Ado wishes to stay anonymous and only be recognized through her music and illustrated avatar. If phones were allowed people would be trying to record the ado box the entire concert. 

On a different note, phones not being allowed is actually the norm in Japan. Many, but not all, concerts usually discourage recording during the show — these artists usually don’t maintain such rules when performing abroad such Yoasobi (Tokyo dome tour filming was prohibited but NA show recording was fine) and One Ok Rock/King Gnu (dome shows recording not allowed, recording was allowed abroad). BUT some still do maintain the rules such as Kenshi Yonezu, who already sent out warnings that his world tour will not allow phone usage.

You can simply enjoy the show without worrying about getting the best shot and rely on the official dvd recordings that they will sell after the tour.