LENS OPTIONS
Each M Frame® style has removable, interchangeable lens options that allow you to adapt instantly to any light condition. Five lens shapes are designed to match the M Frame® and Pro M Frame®, and four lens shapes are available for the Magnesium M Frame®.
M Frame® and Pro M Frame® can be fitted with Rx implants. Currently, implants are not available for the Mag M Frame®. These opthalmic lenses are integrated into the single-lens shields to provide corrective optics, custom ground for your prescription
From the Oakley site. I like that it works that way, at least they're not screwing you into buying a new pair just to get a different lense.