markee174: "Why not release it under GPL and also an alternative license as MySQL does?"
Because obviously they don't think they can sell services & support, or provide enough value-add on top of a truly open source product.
And in the commercial market that Castle's aiming at (embedded devices) the GPL - and other open source licences - have been very succesful. In those markets, the OS without the hardware is generally useless; but many commercial, embedded, hardware products use GPL software with no disadvantages (and sometimes advantages). Think of all those routers running Busybox, a Linux kernel etc. etc.
*Personally*, I'd've thought that an STB maker would want support for their OS, and so value-add services on top of an open source product would be viable. But presumably only if the customer trusts the supplier to be able to supply those services in a timely and efficient manner.
Anyway, I'll stick my 2p in too: it'll be interesting to see how much of a running system can be replaced. There's no mention of the kernel, or most of the core of the OS; so I'd imagine we're most likely to see developers picking up things like !Paint to improve them.