You're clearly making assumptions about how most GMs decide to have something be the case based on your simulationist leanings that I can promise is not what most GMs do, or have ever bothered with; they decide because they decide, sometimes for gamist reasons, sometimes for story based ones, sometimes for reasons they can't even articulate. And when players have learned that somehow its only dark in a place when its going to cause them a problem, they aren't going to forget that soon.
Like I said, we're talking in at least big part about the game that had the main race in play not able to see in the dark but virtually every monster you were going to encounter that you'd ever hit in the dark able to see in it for 50 years. I don't think its a surprise that people's take-home from that was that they needed to get some sweet dark-seeing too should be even faintly a surprise.