And something else: How is this requirement for a resource (fuel) any different from moody dwarves demanding silk or gems when you don't have any?
Because players have a chance to find, produce, or trade for silk and gems (although, as noted, dwarves using
regular forges do not in fact require any fuel.) A magma forge going unpowered is not something a player can do anything about -- the instant it comes up, the dwarf goes mad. The player can't influence the dwarf away from choosing a flickering magma forge; the only possible solution is to never have any flickering forges at all, period, which is silly and unintuitive (there's nothing about a flickering forge that suggests that it could randomly kill people.)
Players are supposed to always have at least some chance to satisfy a mood; there have been many mood options removed or restricted already because they were too difficult for many fortresses to have any realistic chance of satisfying them. Moods are not supposed to be challenges, totally random "This dwarf dies with nothing you can do about it" events.
This is not something that adds difficulty or challenge to the game; this is something that adds random frustration to the game, without giving the player any control over it whatsoever.
What does it being your only good blacksmith have to do with it anyways?
In a non-broken situation, a player has the ability to take good care of their most important dwarves -- they can go to extra lengths to satisfy mood requirements, for instance. Because this situation is badly broken, though, the player is given no influence over it at all -- it is no different from "Stinthad Rithatul had a sudden heart attack and died!", or having Stinthad Rithatul killed by a sudden random bug in the game.
The player should be given at least some way of responding to threats to their dwarves. They might not always succeed, but they should have a way to prepare for them and (if they've prepared) they should have at least some chance to react when they happen. This violates those principals; it kills dwarves at random without giving the player any reasonable way to even try to prevent it.