It sounds like that figure is just an adjusted version of the lie that was used to justify the bicycle helmet law in Ontario. During the hearings for the legislation one lobbyist claimed that bicycle helmets reduce injuries by 75%; when she was later asked to cite a source for the figure she admitted that she made it up but that it "sounded about right". I'm guessing that number was changed to 73% because it would be more credible than a round number. Goebbels was right- an outrageous lie, repeated often enough, will drown out the truth.
FYI some other facts I've learned about bicycle helmets:
1. When Queensland, Australia mandated helmets in the early 90s the number of active cyclists decreased by 25% but the number of injuries increased.
2. A survey conducted in Ottawa found that 28% of cyclists regularly wore helmets, while at the same time Ottawa Police reported that 48% of cyclists involved in traffic accidents were wearing a helmet at the time.
3. CSA standards require a bicycle helmet to be able to withstand a 20 km/h impact on turf. Most cyclists ride on pavement at speeds between 20 and 35 km/h (my average is 27, personal best is 52).
Given these facts, I made a personal decision not to wear a helmet. The biggest dangers of helmet use, especially among children, are twofold: Firstly, helmeted cyclists give the impression that cycling is dangerous (this wasn't helped by the fact that pro-helmet lobbyists and the media were giving the impression that the roads were littered with crushed skulls), which in this era of neurotically overprotective parents translates into less cycling, contributing to the epidemic of obesity among kids. I'm willing to bet that obesity is killing far more people than head injuries ever will. Secondly, there is risk homeostasis- as the perceived consequences of accidents are diminished, people adjust their behaviour accordingly (this phenomenon was particularly noticeable among motorists when anti-lock brakes and airbags became commonplace).
As far as motorcycles are concerned, though, I would not ride without one. Motorcycle helmets are sturdier and speeds are higher so I'm inclined to believe they would offer some protection. I would also wear a bicycle helmet if I went mountain-biking, because there is a high risk of tumbling, speeds are usually low and the ground is pretty soft.
Finally, I would never, ever, try to tell another cyclist what kind of protective gear to use or reject. I only wish the nannies would afford me the same courtesy.