Lots of for-profit commercial entities contribute to open source projects.
The code they're contributing is covered by the same license as the code contributed by volunteer developers.
I understand why we should be cautious about these things, but the current situation is that Valve is contributing a lot and their contributions are open source. Yeah, they're doing it for a profit motive, but not to the point where they're trying to kill open source projects or hide the updates behind proprietary binaries.
Valve is, currently, not being evil. GabeN has plenty of yacht money.