Most of WebOS is based on OSS Linux modules. Most of the UI is written in Javascript. Since Javascript is an interpreted language, it's source is hiding on in the rom of each device.

This isn't the source for the kernel, or linux, but the source for the UI you interact with and some various linux configuration files. If you're familiar with MacOSX think of it has having the source for the GUI but not the underlying OS. It's a bit simple of an analogy, but works.