This musical omertà seems to have started some time before the mid-1980s, possibly as a reaction to yoof bearing transistor radios in the Sixties. I know I started getting Strange Looks when I was sing-testing my filk on the way to work (if they aren't sung, you can't tell if they're unsingable).

I remember a cleaner at work who sang happily while going about his duties. Some spoilsport told him to stop singing; then a couple of weeks later he was told he could sing after all, as it cheered up the place.