The CPR info is interesting. Its been a minute since I did CPR, but for quite awhile when I was still riding an engine it was pretty common. I would say I have been on the scene where we did CPR well over a hundred times and while I would never not attempt CPR when needed, the reality is that the chance of a successful outcome is pretty slim unless an AED is used along with CPR. Of all of the times where I was actually doing the CPR I only had two saves until AED's came along. Since then the number of saves I witnessed rose considerably.
As far as liability for treating a patient those of us who are trained are actually in a worse position since we are expected to follow the standards of patient care that we are trained to. If we do not than we can be liable if we are "grossly" negligent. Defining gross negligence is what lawyers get paid for.
So, I would not be concerned about providing aid to someone who needs it. Most states have good Samaritan laws and unless you are foolish enough to play Dr. House and try to do something like a tracheotomy you will be fine. And frankly I'd rather take the chance of providing care than live with the guilt of not helping someone who needed it.