And so is a drunk, dangerous driver.
The law is designed to encourage us to wear our seatbelts, because there is sound and solid evidence they save lives. It's for our own good. When the first seatbelt law was introduced, deaths fell 40%. Simple statistics, repeated all over the world.
When helmet laws have been introduced, similar effects simply haven't occurred. A seatbelt is not the same as a bike helmet, they are two completely different items. The correct comparator for a bike helmet would be wearing crash helmets in cars, and nobody suggests that you're too blame or negligent if you don't do that.
And while it's true that seatbelts save lives, if you're the victim of an accident caused by somebody else, well that person shouldn't have caused the accident.
If I punch someone who has an 'eggshell skull', I have to face the legal consequences if they die. And if a motorist kills someone wearing no seatbelt they should likewise face the consequences.
Seatbelts are worn to protect the wearer, NOT the maniac driver who ploughs into you.
When you do wrong unto someone, you have to take them as you find them, simple as that.
Other equipment might be relevant - a cyclist or car with no working lights would potentially be negligent and reduce fault for the other party, because they would be less able to see you, but personal safety equipment doesn't fall into that category.
The bottom line is it should be simple to point to bike helmet laws and show that they had positive effect as the seatbelt laws did, but that's simply not possible, so not only is my personal safety equipment my choice (albeit that in the case of seatbelts, if I don't wear one, I am breaking the law), but in the case of bike helmets there isn't anything to prove that bike helmets even save lives.