I've noticed that in folk circles, people tend to sing along, probably because of the tradition of hootenannies and song circles. In bluegrass circles, people do not tend to sing along unless they can find a hole in the harmony and sing something different from their neighbor. It's part of the style in bluegrass to not step on the next person's part (instrumental or vocal), so each individual instrument and voice can be heard.

This, by the way, causes no small degree of contention when folkie meets bluegrasser in the same jam.