I've toured with one professional gospel choir of 50 people and we were asked to sing on every flight. We had secular stuff specifically for things like that, but choral music at large is inherently religious, especially in classical music when most text comes from sacred music. We'd sing one SHORT song (1-2 minutes) and then be quiet. Every director I've ever had is uncomfy with it. If a group is jazzed to invade the public with their music, I'd definitely not want to be a part of it.
I also agree that it's not considerate of the other 150 people who paid to be there and in quiet. But point is - they were likely asked to do so because a flight attendant thought it'd be a fantastic idea lol.
I think there's a difference between a professional gospel choir on a flight being prompted by the flight-staff to perform as a special treat vs. impromptu Youth Group Bobby and co. trying to flash-sing and proselytize with a contemporary Hillsong United piece.