This whole conversation seems to flow around the fundamental idea that folk music is divided between performers and listeners. Somehow, the community aspect of music seems to be forgotten. Not that people have to sing along on every song, but unless the singer is singing at the top of their lungs, I am always pleased to see it.
(Besides, if I forget somw words, I know who to ask.)

Over the years, I've seen performers who resent people singing along on the chorus. Hey, dummy, that's why they're called choruses. The only problem I've ever had is in a health care center where I sing once a month. There is a woman there in a wheel chair who used to sing leads in her church choir, and when I do He Knows How Much We Can Bear, she sings along at the top of her lungs, with prhasing very different than mine. At first, I found it very distracting, but Now I see how completely engrossed she is in singing the song with her head tilted back and her eyes closed. She is transported back to a more joyful time in her life and her face lights up. Who am I to begrudge her singing along, even if it is distracting?

Other than that one situation, I see people singing along on songs they know that don't have a chorus. They're singing along softly, and I ma happy to see it.