As one who has sung a long time in a big chorus, I'd have to say I think a reference to a chorus or orchestra as justifying reading a song off a page is a red herring.

Composers are extremely precise about exact dynamics, duration of notes, changes in tempo, etc.   And conductors add another layer to this.   You have to be watching the conductor constantly-while also observing every little direction given by the composer in the music.

It's a far cry from singing a song in a singaround--where you are the conductor, free to interpret the song as you see fit, especially on verses--unless you're bound by the straitjacket of "group singing"--every person singing every word.   Yes, in singing a classical piece you need music in front of you--with very few exceptions.   This is vastly different from the situation in a singaround.