> Hard to understand their stance on keeping Source closed

Forget Source, what about GoldSrc.

... which was based on Quake which is now GPL.

There's Xash3d which lives in a kind of licensing limbo because it was developed mostly from scratch except for some header files which could now be replaced by GPL QuakeWorld ones yet the current Xash maintainers can't do anything about it because they're tainted by non-clean-roomness.

Even if the above issue was solved, this is about the engine; the HL1 game code (the "SDK") is open but licensed by Valve in a way (MIT-ish permissive with a non-commercial clause) that is incompatible with the GPL.

This makes both legally undistributable as binaries together.