If it's too dark you can't see it. Maybe they thought it safer to default to Auto in case you were riding at night in Battery Save and when the screen turned on you can actually see it? Dunno, just a guess. Why does Garmin still have Virtual Partner is my biggest question...
When battery save mode is onz you can't adjust brightness, it sets it to auto.
On that note, why if your trying to save battery would you want the backlight on at all Garmin? Idiots.