If there is an understanding that the singer doesn't want you to sing along, then this should be respected of course.

Manners and sensitivity do play a role here.

But the best communication for most people is a sense of participation which if encouraged by the performer enhances that performance.

I have been bored to tears by some long-winded, pretentious, self-important singers
who insist on a "concert platform" for their "precious" offerings.

Folk song academia is a disease which is not limited to information but a great deal
of attitude.