I'll elaborate on my previous post, for fear of being a victim of my own suggestion.
Compare the two possible situations that Becky mentioned, and put yourself in a first person perspective;
UK:
"Ohnoes, teh person has fallen off their bike, I'll run over, check they are OK, and call an ambulance if they need one. That way, they can get better in hospital."
USA:
"Ohnoes, teh person has fallen off their bike. However, I can't be sure that they have insurance, so instead of getting myself involved in some sort of potential scam or putting myself in a situation where someone needs healthcare to live but isn't covered, thus making me feel like a lesser human, it'd be best for me to not get involved."
OK, the so the dialogue isn't the best, but it's pretty much what I was shooting for.