Actually, in the US [0] it's the default that you can be sued for injury or wrongful death if you attempt to assist. Some jurisdictions have Good Samaritan laws [1] to fix this very issue, though even then it sometimes only extends to trained personnel like emergency responders and doctors.
[0] And possibly other places, but I'm not familiar enough to say.
I do not understand why you would hold such a false impression. Anyone can indeed attempt to sue you for anything, however that says nothing about their likelihood to succeed.
Say I am untrained in any rescue or medical procedures. I witness a car collision, and a rider was ejected from a vehicle and lying next to it. I proceed to move them due to perceived fire hazard, however they in fact have a broken neck and die due to my moving them before stabilizing the neck. If it could be reasonably argued [0] that the person would have lived with proper intervention had I not intervened, why should I not be liable for that?
In fact, without a Good Samaritan law in effect, I am potentially liable for the rider's death. I can be successfully sued by the survivors. The whole point of Good Samaritan laws is to reduce or eliminate this liability to me in exactly these scenarios.
Now, if I do nothing, I am likely to only be liable if there is a duty to rescue [1].
[0] As in, well enough to win a criminal or civil case.
[0] And possibly other places, but I'm not familiar enough to say.
[1] http://en.wikipedia.org/wiki/Good_Samaritan_law