Tesscaline has put forth a decent question. How are these things more risky than other activities in which 16-year-olds are permitted to engage?
Tescaline has also refuse to argue honestly.
We can't regulate away every risk a kid might take, and we can't criminalize kids themselves for most actions. What we do is put the burden on the adults in their life. If their parents knowingly let them engage in high risk behaviors, they can face legal troubles.
If they are with any other adult who is facilitating what they're doing we tend to hold them responsible.
As for other risky behaviors,
Hunting
http://www.michigan.gov/dnr/0,1607,7-153-10363_14518-32244--,00.html
Soccer
http://sudburysoccer.org/First_Aid
Bike riding
http://www.helmets.org/mandator.htm
Or for anything else, just look at state level child endangerment law.
http://www.shouselaw.com/domestic-violence273a.html
And so on. Whenever a minor takes risks with an adult, generally some level of law is in place to minimize those risks, whether it's national, state or local.
The question isn't about these risks being necessarily greater, but being unmanaged. If your kid is training to be a gymnast, her coach is trained in first aid, by regulation. A first aid kit is on hand. They'll have liability insurance that covers her medical bills if something goes wrong. Their equipment will have been inspected. Your kid will be trained in proper safe use of this equipment, and if not, then the teacher may be guilty of criminal negligence. There are all these regulations in place to prevent injury, and to make sure that if an accident does happen, it's handled in a way that minimizes damage as much as possible.
So I'm not saying that BDSM is some great exception. I'm saying that
some BDSM practices that carry risks for minors should be regulated, just like we regulate all those other practices. And those regulations often include prohibitions when adequate safety measures aren't viable. In Michigan, a kid under 17 can't go off hunting by himself.