The Nintendo Wii U in-store demo booths maybe running a modified version of the Ubuntu operating system instead of the Wii U itself.
One user on Reddit obtained a snapshot of one of the systems that hadn't booted correctly because it was missing a "USB key". Instead of showing the games available to try out, in this case Rayman Legends, it displayed a screen for the Ubuntu OS.
Other reports are suggesting that the demos are running off a separate computer, housed underneath the dummy Wii U shell, with the GamePad hooked up to that instead.
The use of a separate base for storing and running downloads could be a security measure to prevent Wii U consoles from being taken prior to release, and also to limit functionality, preventing users from accessing the Wii U's system menus.
It's uncertain what system/core the actual Wii U runs on whether or not Nintendo has built the console's functionality over a Linux base.
Thanks to Linkyshinks for the tip.