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.
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.