I am of the opinion that the Hammonds were prosecuted not for starting a back fire but for letting it get out of their control.
Dry grass and brush when combined with a little wind... by their nature get out of control almost immediately. I cannot comment on the details or wisdom of the Hammond’s actions, because I do not know enough of the specifics. But I have had enough experience that I do know that fighting these types of fires require good tactics and strategy to get them under control. If you have people acting outside of the command structure that it can cause dangerous situations regardless of their motives or intent.
I also have fought wild fires and know full well that:
1) Fire wants to burn
2) Fire is a stubborn thing
3) Most people VASTLY over estimate their ability and skills when it comes to containing a fire.