Having known folks who trained for that mission, it's not particularly easy or safe. External cargo missions over water are relatively easy and safe. Hauling external cargo over a wildfire, with visibility nil and extremely unpredictable air, is much more difficult, and the price tag of getting something wrong is a crash landing in the middle of a fire.
This is a training failure on the Navy's part--units based here in the Southwest should be 100% proficient in this from the get-go.
There's also a monetary angle (as there always is). Most of the firefighting aircraft are owned by private contractors.
They get more money for a big fire than for a little fire. So they have a perverse interest in generating large fires.
They have a lot of influence with the CDF (because they work for the governor). They argue that federal law requires that the state pay the military, and that it will cost a lot of money.
Unlike the fixed wing aircraft tasked for fire missions, helos don't have to swoop into the canyons and pull up to clear the ridge. They just get over the fire and drop, even high over. The fire line is almost always visible from straight above, since the thick smoke is blowing away down wind.
How much does a water bucket cost to manufacture and store? It's criminal that these helicopter assets are sitting on the ground idle 15 miles from the fires.