"Aid to the Civil Power" requires a lot of justification. All the civil avenues of performing the task have to be exploited, otherwise somebody will bleat that the Army has taken his job on the fire lines.
Wiring for a bucket would need extensive testing by ARDU, and then every aircraft of that type would need to be fitted, the crews trained, blah and blah. Military aircraft have enough stuff in them without extra wires for a "just in case" exercise.
Mil aircraft are designed to START fires, not put them out.