It makes sense though. If I watch a video someone grabbed of a band playing a song I like it is not going to take away from my enjoyment of seeing it live for myself. In fact being there personally where you're enveloped in the music is an experience that is both different and improved from watching it on a phone, computer or tv.

However, for a comedian if you know the joke & punchline the element of surprise is taken away so it is not going to be as funny and if it is widely known the crowd reaction is going to be seriously diminished.

Plus the big popular comics usually try out material at smaller clubs before going on tour and have a vested interest in not letting a work in progress getting out there before the real money making venture.

However they should use the magnetic lock bags instead.