I think BMW are yanking your chain. If you have say, a suspension failure that can be directly (but unlikely) attributed to the tyre you fiteed, then they will deny warranty. However, a tyre is a wear and tear item and you can choose to fit 'better' tyres if you wish. I don't think any court in this fair land would rule in favour of BMW if they denied a warranty clain on say a wiper motor failure just because you fitted different tyres.

Hope this helps

Robin the Technician