Just me shooting from the hip, but I would imagine duty to act laws come into play here. it's one thing to ignore an emergency when your off duty in your POV, but it's quite another to ignore a flagger while in an ambulance with lights and sirens. You might not get sued, but you can get dragged into an investigation.
Then there is the whole "court of public opinion" issue. You might not be sued, but the headline "paramedic drives by baby not breathing" with the sub-heading "doesn't even stop to help, was late to a non-emergency run" probably won't go over very well.
Hypothetically speaking, if you did have a stable patient in the back, why would you not stop and render aid? It's good PR, your patient isn't dying, so another 10 minutes won't affect his outcome, and if the patient is sick & dying, you can scoop and run the both to the closest ER. Obviously this wouldn't work if you are transporting to a sick and dying patient.
Speaking as a dispatcher, I would want you to tell me about the incident and I will tell you to stop or continue to your original assignment. I might have it already, or I might not. I might have a unit that will end up closer to your original job, at least compared to the flagged down call. If you are going to a toe pain, or other non-life threatening dispatch, I might tell you to stop, because the toe pain can wait a couple more minutes until another unit arrives. Best part about all this: if something bad happens, you can always say, "I advised dispatch, who is aware of all calls in the system, and they directed me to take that action. I was only aware of one call, so I assumed they knew the call was of a more pressing issue."