[bsimpson]

I believe the root constraints are a security thing. They don't want you to charge your phone in a dodgy USB outlet in a public space (e.g. an airport) and be compromised. Making you flash your phone to get root is meant to prevent people accidentally exposing themselves.

[BatteryMountain]

Google can provide a non-rooted system image out of the box for safety, but nothing stops them from providing a clean & rooted image for download that I can flash onto device if needed.

The security model should already be tight enough so that a dodgy usb outlet does not compromise the core system (rewriting system images).