Sensationalism alert. This is a soundbyte; here's more context and less shitty journalism: http://www.muktware.com/4042/richard-m-stallman-steam-good-g...
Why not link to Stallman's actual post? https://www.gnu.org/philosophy/nonfree-games.en.html
He has a valid point. You have more freedom if you run proprietary software on free OS than when you run proprietary software on proprietary OS. As a bonus point presence of Steam on Linux will probably bring fixes to free software graphics and sound components.