So what exactly is his problem? He should be glad that Valve is doing this for Linux. No one is forced to pay for anything. If Linux users want to pay for the games, then they will. If they don't, then they don't. I don't see the issue here.
He believes that software should be like human languages. Free for anyone to learn, modify or invent. If you invent a word and it gets notoriety you don't "own" that word as anyone can use it for free, they can modify it, say their modification and show others how to say your word. No royalty, no "word police" telling you that you are unauthorized to use that word without paying a license fee.
So when Steam launches on linux and it sells games and locks them behind digital rights management (you need to login to access your games after paying a license fee to play them) it goes against his principles that software should be open and free for modification and redistribution.
He is against closed source software from a fundamental freedom paradigm where by he believes all people should have access to the source code of software so that they can learn from it, modify it, improve it, fork it, redistribute it etc
I guess he could be described as the ultimate socialist I don't think in his world it is really possible to own a piece of software. I don't agree with his views, I think developers should have the choice to release open or closed software and deserve the right to decide who can and cannot use their software based on any criteria they like including a monetary one.