Ludwig was right (several pages ago), a helmet is unlikely to stop a fatality but could make a non-fatality a lot less serious. Falling off and smacking your head on a kerb at 15mph would be a lot less damaging with a helmet on - nothing to do with being crushed under a bus.
By your logic, Gromit, the problem is that you'd be more likely to fall off and smack your head at 15mph in a helmet, because you behave differently. In that case, in order to make cycling even safer than it already is, we could remove the saddles, cut the brake cables and take the tread off the tyres, thus forcing people to ride even more carefully than they already do. I doubt it would breed many Olympic champions though, and that was maybe Wiggo's point - he wants cyclists to be able to ride quickly and safely, not just safely.
Still, if cyclists don't want to wear helmets I guess that's their choice. It's as I said earlier - do we have to legislate against people's own crass stupidity? In certain cases we do - seatbelt laws for example - but often we don't.
I agree that the issue is really how responsible motorists, cyclists and pedestrians are for their own actions towards other people, and the "safety equipment" is almost a separate, parallel issue. But a cyclist with a well maintained bike, decent lights, a helmet, a bell and not listening to headphones comes across as a lot more responsible - for themselves at least, if not for others - than one on a shoddy bike, no helmet, no bell and listening to headphones.