Microsoft's main source of control over OEMs is the logo program. All laptops that want to get the "Designed for Windows" sticker (logo) have to pass a minimum bar set by Microsoft, which includes things like "must boot within X seconds" or "must sleep within Y seconds", "must output the correct audio frequencies", etc. My guess is none of the logo tests invoke the accelerometer which is why this got missed.

The logo program is great in theory, but in practice it doesn't work too well since consumers don't care about the sticker. Therefore, if Microsoft makes the requirements too high, OEMs will simply stop trying and just sell their laptops without a sticker.