Licensing? What do you mean, licensing? Other manufacturers are free to take it. It's all open source.
Why would you not need to license it? Steam isn't open source and Steam, the trademark, is owned by Valve. If we were talking about a standard distro like Fedora, no, I guess they wouldn't have to license it, but we aren't.
If someone starts selling completely shitty "SteamOS PCs" without licensing, it'll hurt Valve more than the no-name Chinese PC manufacturer.
Licensing -> Valve can dictate minimum specs and QA requirements -> Good for everyone.