When you attended to the emergency you obviously had no intent in gaining any monetary value out of it. You certainly did not book and board that particular flight with the hope to get some form of reward for caring for a would-be ill passenger. The offer to upgrade you (or give vouchers) came after you had delivered treatment.

The patient did not pay or give you anything for your service. The airline solely chose to compensate you for the inconvenience caused to you as a passenger. Though I can see how you might have been exposed liability wise should the patient had complications post treatment.

I find this very interesting.