I think there is a very small % of CC that enforce or even warn pax about the SB sign. Most of the time it appears that the CC look the other way and pretend not to see the pax. I have even seen pax walking around when the CC have been told to sit down.
Have to agree with gtseraf, I don't want some idiot smacking into me because they wanted to get something out of their carry-on in the over head.