Valve explicitly says you cannot use that version of the Source SDK to make games on Steam, yet Classic Offensive did just that and subsequently got blocked for it.
Play by the rules and you can public just about anything you want mod-wise for Source.
There is no company that comes close to allowing the sheer content and modability than Valve.
They literally give you a full SDK, near full editor tools for both Source 1 and Source 2. Ability to publish games using some of these tools and for free, host the Steam Workshop and its likely PB's of modded content.
I don't see Activision given rights and modability to CoD Games, they DMCA instantly.
I don't see EA/Dice letting people use the Frostbite engine.