I saw these riding in to the city from Allston today. It wasn't until I hit Kenmore Square that I could slow down enough to make out the writing, and then I was just confused.
My guess is that the intention is to say something like, "I shouldn't have to wear a helmet, it's the absent-minded and terrible drivers of Boston that ought to be more conscientious and courteous to cyclists and then there wouldn't be an issue." If a biker gets hit and suffers a head injury, then it's his fault for not being properly outfitted. Of course even if the only way for a cyclist to sustain a head injury (it obviously is not) was in a car-bike accident, the key word is accident so this line of thought is all wrong. I may be wrong, but people usually don't refer to a "victim of an accident" as victim usually implies some sort of malicious intent was wrought upon you. Though in my own cycling I've occasionally been suspect, I've never seen or heard of anyone being intentionally mowed down by a car.
Accidents happen, and will always happen, even if you want to argue that the effort ought to be spent getting drivers to improve their behaviours towards bikers (which, incidentally, I would). People who don't wear helmets are dumb; it's just a potential risk for which there's no real reason not to be prepared.
