Mechanical warranty to be cancelled because of glass

I have to replace the windscreen on my Nissan Quashqai. The vehicle is still under warranty. I have to replace the windscreen because of a crack. The insurance informed me that Nissan requires that the windscreen be replaced with a Nissan Genuine part Windscreen, otherwise I will loose my mechanical warranty. The Nissan windscreen costs 3 time more than a normal windscreen, most probable from the same factory.

My question: How can you loose a mechanical warranty when the glass (nothing mechanical) is replaced?