In past vehicles I've owned that had automatic switching between day and night modes (ford, gm, saab), I usually found that they were tied to the activation of the headlights. If I wanted the headlights on during the day, the display on the nav would go into night mode and I couldn't change it. I usually turned off the auto-switching feature.
I have no idea why BMW designed theirs without this option, but I would surmise it was either a design choice for reasons like the above, or it was an integration feature with other car systems that they couldn't get done, while also providing manual control of the ability. Who knows though.
