You can get arrested for any kind of physical behaviour between a fellow person, all it takes is a mistaken thought, a sensitive victim and a sheer amount of bad luck.
Can it happen? Yes
Will it happen? Maybe (Probably not)
A lot of First aid is physical and not the soft kind of physical. It's an open ended book in which if you did somehow piss off that guy you just saved the life of and he holds a good grasp of law (or holds money), he can essentially use that against you.
Ethical principles are kind of loose when it comes to intention. You could have had the intention of saving that poor old ladies' life (and you did), but you broke her frail ribs in the process so she wants to sue you. That can go wrong fast, but that's the fundamentals of first aid, you have to act.
If so, should I not do anything at an accident scene or is there guidance as to what is acceptable?
When I was taught first aid, I was taught that it's place in the world is required and you should do it, but you have to learn that there are definitely negatives available for interpretation... Americans are very connected to their freedom and self preservation, but I think you should act the same way as you would in your home city.