Completely agree. I have never bothered cleaning mine - the first time I actually touched I realised it's some sort of plastic which my gut told me was very sensitive. Mine is a bit stained, but frankly I have iDrive always on which hides all of it.
Demineralized water and lens cloth... Gee. Though I realise they can't just put a glass on top of it as that'd be a safety hazard, I'm sure there are tons of proper plastic covers these days which don't get scratched by the mere gaze of the driver.
You should write to BMW Corporate. I've written to them twice and they've replied quite professionally both times. Of course normally you'll get a standard 'thank you for... the cleaning procedure is described in... we remain at your...' boiler plate reply, but at least you can insist they seriously look into improving the current situation.