I don't think there is any exact canonical reasoning behind the term, but from speculation, I believe the following reasons might explain the term used, specifically if we go in the context of Generation I games (the originals):
HMs are rare - While obviously there exist several copies of the item, these cannot be bought, nor found lying somewhere for the player to pick up. You would rather have to accomplish something specific to get them (HM01: Help the S.S. Anne Captain, HM02: Talk to a secluded girl, HM03: Reach the end of Safari park, HM04: Give back the Safari Zone Warden his lost gold tooth, HM05: Register 10 Pokémon on your PokéDex).
HMs cannot be forgotten - There were no move deleters and Pokémon knowing HMs could not be bred. This must mean that there was something rather special with HMs that make Pokémon unable to forget them if they learned it.
HMs are reusable - Compared to TMs, these will not be 'consumed' after being used, so you can use it as many times as you want without having to worry about looking for another copy of the item.
Of course, these points now are no longer that much valid when applied to the subsequent games. HMs can be forgotten easily with the Move Deleter, by leaving the Pokémon at the daycare long enough so it learns another move that overwrites the HM and some HMs can even be forgotten through the usual course of gameplay.
Waterfall (HM07) can also be found lying in a cave in Generation II games (an ice skating puzzle has to be solved, but it is not obtained from a person) so the validity of the point raised above is no more that strong. Still HMs cannot be bought.
And of course, there have been HMs moving to and from the HM status. Whirlpool is the first one to do that, becoming a normal move in Generation III games and becoming back an HM in Pokémon FireRed/LeafGreen.