[onli]

Completely not legal. Maybe contract law is bloated enough in the US to give valve that power, I'm pretty sure it isn't in the EU. Valve can't decide how ownership works, not in all jurisdictions.

[cbaconnier]

Worth mentioning that you don't own the games, you buy non-transferable licenses to play the games. Therefore I think it's perfectly (and unfortunately) legal.

[slowmovintarget]

You can gift the license to someone else, can't you?

[Volundr]

You can buy gift licenses from the store and give them to someone. You can't just gift a game from your library to someone.