Lego's argument always struck me as absurd. Imagine Ford doing the same thing: "Please refer to our vehicles as 'Ford vehicles' or 'cars', and not 'Fords'. By doing so..."
If the company doesn't take reasonable actions to protect the mark, they lose control of it.
Ford doesn't worry about losing their brand. Lego does. So Ford regularly uses the brand as a noun itself, Lego Group asks people to not do that.