Chris Boardman has the right idea. "If you want to wear a helmet, wear a helmet".
Thing is, though, we don't say that about other health and safety issues. We don't say "if you want to eat sugar, eat sugar" - we actively and strongly discourage people from doing something that the evidence shows is harmful. At the very least we tell people what the evidence is in the hope they will be swayed by it.
Sometimes we go too far and sometimes not far enough, but I like the principle that we are encouraging a society where decisions about risk are based on evidence.