Huh, that's an interesting result, though perhaps not entirely unexpected. Another aggregate study can be found at [1].
I still suspect that the difference in risk between biking without a helmet and with a helmet is significant, and that the studies aggregated likely had a majority of riders using helmets (this is entirely based on subjective experience of US cyclist helmet-wearing probabilities, so let me know if that's wrong).
I'm not sure, but I do know that in the Netherlands no one bikes with a helmet, and it's considered very safe: I took the bicycle to school ever since mid primary school (we don't have school busses or anythig like that.) The only people that I ever see wearing helmets on bicycles are North American tourists.
I guess it depends heavily on the locale - I live in Portland, and while we're one of the most bike-friendly cities in the US, going helmetless would be risky at best, though outside city limits wouldn't be as concerning.
I still suspect that the difference in risk between biking without a helmet and with a helmet is significant, and that the studies aggregated likely had a majority of riders using helmets (this is entirely based on subjective experience of US cyclist helmet-wearing probabilities, so let me know if that's wrong).