Now to get back to where this topic started from: Where I am at, there are no laws that I am aware of that require an on duty EMS crew, that is not 911, to stop and assist in that situation. Most of the EMS systems that I am familiar with have policies that require EMS crews to stop and assist, some of them require transportation, if necessary, but usually they would have to wait until an on-duty 911 EMS crew to arrive at the scene.
Even if there were no such policies, common decency would essentially require them to stop and assist. If I am off duty, not in my ambulance, then I have no such requirement. What I would do in that situation, as an off-duty paramedic, is simply act as a good observer and call 911.