Thats not entirely true. ARS and another company are authorized by Glock to make the alterations. The warranty is only voided if the reduction is the direct cause of the failure. Supposedly, these modifications have been done to a lot of Glocks and there haven't been any problems.
From the ARS website (I'll admit that it doesn't say free repair though):
[ARS will modify your Glock grip to decrease the overall circumference and change the grip angle to a flat style (like a Colt 1911 with flat main spring housing). This smaller and more comfortable grip makes it easier to reach the magazine catch, shortens the distance to the trigger, gives you more natural sight alignment and better recoil control, mitigates potential slide-bite on large hands, and improves accuracy potential through a smoother trigger press. This modification does not affect any moving parts and is widely accepted by law enforcement agencies. It makes the weapon safer by giving you more control. Glock has stated "We will continue to service and repair all Glock pistols regardless of what after-market product has been fitted to the pistol."]
