As a surfer and mountain biker who used to have a pet rattlesnake and is planning on free-diving with great white sharks next year, I've done and will continue to do my share of life-threatening things in life. But for many adrenaline junkies, what their lives would become without these sorts of risks is far, far worse and more life-threatening in a MUCH more pointless way.
The thrill that adrenaline rushes like this provide is nothing more than the result of a massive expulsion of dopamine by the hypothalamus. In very simplified terms, dopamine is released as a sort of Pavlovian response to a reward, whether perceived or real. In drug users, especially users who have not entered the addictive stage of drug use, dopamine release can happen as a result of the "reward" that is associated with whatever good feelings that drug use may result in. Although not always the case, drug addiction can occur as the user searches for that dopamine surge that initially accompanied drug use but is no longer there.
In adrenaline junkies, the reward can be anything. It can be surviving a long tube ride at Pipeline or earning a shitload of money after a great season, it can be riding a vicious Brahma bull for a full 8 seconds. Or it can simply be completing a PR snatch or clean.
For many people, that need for the dopamine surge is unavoidable. You want to achieve whatever feeling it is that caused that initial rush of chemicals out of your hypothalamus. Obviously, it is much preferable to achieve this through athletic endeavor rather than drug use. With athletes, the reward (on a purely chemical basis) is much more gratifying than drug use, and therefore more addictive. I've been to the extreme in terms of drug use and I like to think that I've experienced similar rushes from athletic performance, whether it be playing baseball at the DI level, surfing on waves that test the limits of my skills in dangerous waters, hurtling down a mountain on a bike at 55mph or even just pulling a PR for deadlifting in the middle of a gym filled with beautiful women. With athletic performance, the reward is enhanced by the extra blood flow that occurs and the accompanying surge in all sorts of nutrients and chemicals that doesn't happen with drug use.
Any sort of creative outlet may lead to this. I don't know how valid the following theory is, but I do know that Miles Davis experienced at least two distinct periods of heavy drug use in his life. Both of these periods came after extended periods of extreme creativity in which he literally changed the entire face of not only jazz, but music in general. The more famous of these two periods was his acute cocaine addiction in the latter half of the 1970's, which followed his prolific electric phase from roughly 1968 to 1974. This may be a result of searching for a similar "reward" through drugs after the initial creative surge has been exhausted.
The point here is that adrenaline junkies may very well be better off putting their lives at risk through their sporting achievements because the alternative method of gaining whatever "reward" they seek may come from drug use instead.