When I've slept on a bed without a headboard, my pillow tends to fall off the end of the mattress as I push it around when I sleep. The headboard acts as a fence to keep the pillow on the bed. To me, a mattress sitting on a box spring held up by a frame doesn't look so much cheap as incomplete. But since the bed is typically out of sight in a bedroom, people generally don't sit around gazing at it.
The resentment behind the terms is scarcely concealed and bursts forth especially when commenting on those who are both smart and educated — a combination that is totally repulsive to many. I see that, but I don't understand it.