Yes, at least in CO there is a law that requires you to render aid unless there are others already there, but I don't think they require you to stop as a witness.
We do it to ourselves - People were getting sued for rendering aid incorrectly so they just quit stopping. We had to pass the Good Samaritan law that exempted you from liability for stopping and rendering aid just to get people to start helping again, but you were only exempt if you were never trained in medical response.
Serve as a witness, and you can get a supboena to testify with no regard or compensation for your job or where you live out of state.
I don't think that its that they don't care, they just don't want to get involved. It's just not a very friendly world anymore.