Having the gear down would help make the aircraft more stable as far as airspeed goes when dealing with updraughts etc. think having the drag to help prevent flap overspeeds etc. not to mention the un-silenceable warnings not having the gear down
if an emergency occurs then it’s simple to drop the water and instantaneously improve your performance and then clean up.
im not sure about the FAA reason but having flown quite a few fire seasons as a bombing pilot (albeit in the rotary wing world) and now a jet pilot, that would be a practical reason for it.
hopefully someone who actually flies that jet in the role can post on here with a definitive answer