Joe,
I think you hit the nail on the head when you pointed out that many people think singing is performing instead of communicating--that it is somehow different from talking (which most of them do without being Martin Luther King), or writing (which most of them do without being Oliver Wendell Holmes), or telling a joke (which most of them do without being Drew Carey). How to get around that? I don't know. I think its partly a matter of being so much out of the habit, that what they've forgotten is not how to sing that they want to.
I think you just gotta keep em doing it until they realize how much they miss it.
Jack