why is it as soon as PCs realize a problem can be dealt with a forest fire, DMs pull overpowered NPCs out of every cranny from thinking outside the box
Because boxes, as a rule, are flammable?
As a DM, I would equate the intentional destruction of a natural ecosystem with an evil act, but then again I like forests. If the forest itself was inherently evil, that's a different matter.
And then again, in the rainforests of Belize some farmers intentionally slash & burn a tract of land to create a "milpa", a garden fertilized by the ash of the fire.
".. a forest suddenly obliterated by a fell magical attack might remain as a ghostly grove populated by lingering spirits not even completely aware of their own destruction." - Savage Species
In the end, as the DM, I would not stop the PCs from burning down the forest. I would however, bring the charred remains of the wood back as a ghost brute, by night, and have the local village blame the PCs directly for its genesis. But I'm fun like that. As a contrast, the charred ashes of the old forest fertilized the birth of a wondrous new forest, to be enjoyed by all during the day.
"Bruce Wayne: The bandit, in the forest in Burma, did you catch him?
Alfred Pennyworth: Yes.
Bruce Wayne: How?
Alfred Pennyworth: We burned the forest down." - The Dark Knight