That's a common finding in helmet safety studies. It was used as an argument against hockey helmets in the 1980's. The reason is that people who are wearing helmets often think the helmets provide more protection than they do, and are more reckless. In the case of hockey, this means they're more willing to go charging toward the boards, or deliver a hit at higher speed. In the case of cycling and skiing, it likely applies two ways, it makes people feel invulnerable, and people that use helmets in those sports are likely traveling at higher speed or are more intense participants.

There are very few people that crash badly while riding a beach cruiser to the ice cream store, and those same people are less likely to even own a helmet.

But that doesn't say that helmets don't provide additional protection.