1. They don't want their show recorded.

2. They don't want their show "leaking" on the internet ("loss of ticket sells")

3. People using their phones "ruin the experience"

Realistically it's due to:

1. They say libalious things. (Aka Hanable Burris talking about Cosby before formal conversations were had) Also, they say things that upset groups of people who try to "boycott." Aka Louie and the Parkland shooting joke./"Cancelled"

2. They are pissy about bad recordings going up on the internet and being caught behaving badly. (https://www.nydailynews.com/entertainment/dave-chappelle-fan...)

3. It's there to protect notorious joke stealers from being caught in the act.