Engineering director David Burke says this requirement is meant to provide a "very consistent" experience; you won't have to grapple with different controls just because you're watching TV in your bedroom instead of the living room. The shift in policy also lets Google manage the update process, which should be as "automatic and seamless" as the company's Chrome browser. Theoretically, you won't see the haphazard upgrade process common to non-Nexus Android devices, where updates sometimes arrive months after their official launches, if they arrive at all.
The move is likely to please fans of Google's approach to interface design, or those who simply want to try new features as soon as possible. However, it's a sharp break from Google's mostly laissez-faire attitude to Android on phones and tablets -- car, TV and wearable makers will primarily have to distinguish themselves through hardware if they want to use the bulk of Google's services. If they insist on their own control layouts, they'll have to take an Amazon-like approach and build (or find) replacement technology. The strategy likely won't preclude you from loading custom software after the sale, but the Android you see in stores won't be quite as diverse as it once was.