I'm sure it is policy for passengers not to get off a plane when a flight diverts to re-fuel (most of these flights aren't on the ground long enough for pax to deplane and then re-board). They will always open the door to get paperwork (i.e. fuel information, new flight plan, etc.). I'm sure you technically forfeit your ticket when you do get off (mitigated by the fact that the agent re-booked you via phone).
As for the legality of the issue, I can't imagine there being any serious possible consequence in your case. Certainly if it was an Int'l flight, there would be issues, and perhaps also if you had checked a bag.
In your situation however, if someone wants to get off an aircraft, and the door is open, I can't imagine this is a serious problem; especially considering the PAX Bill of Rights legislation.