its a blanket rule, which has almost no exceptions. So there are some silly parts. One of my kids is in band and the school uses YONDER pouches. They have had to dig out some really, really old analog tuners to use. They have a fraction of the capability of a $4 IOS app, but the kids are supposed to keep their phones in a special sleeve with no exceptions... (so many kids break that rule, or throw an old dummy phone in the pouch)
Tuning by ears is an important skill for musicians, learning that is beneficial. For example, you cannot rely on apps to tune your signing voice during performance.
