Exactly. If it was going to take a bunch of programming and special cases to make it work like it does currently, I could see the devs saying "Nope, not a big deal".
But you'd have to be insane to say "Well, it works well and is fair right now, but let's change it to be less fair, with additional code, so occasionally players get killed by performing the absolutely necessary action of going down stairs".
Since monster energy is completely invisible to the player, nothing that happens with monster energy can be unintuitive. What would be unintuitive is just dying sometimes when you go down the stairs.
The game would be... not a game without the monsters, so uh... you might as well say "starting crawl.exe is the #1 cause of player deaths, so lets just delete dungeon crawl" Getting killed while going downstairs is not a source of fun for anyone, and frankly, if you think the game should do it, I would prefer that you never participate in GDD again...
Also, here is the most important quote from that design document people love to quote:
Ideally, a game like this would be really challenging and have both random layout and random course of action, yet still be winnable with perfect play.
We know that's just not possible, but it is the ideal, and thus, it completely goes against the design philosophy to purposely add a new source of unavoidable deaths.
Plus, could you imagine your first descent to zot5, then "You go downstairs, the curse toe torments you, the electric golem hits you, the electric golem hits you, you die"
I certainly would never play the game again...