As RMS mentioned, the danger is not in Linux becoming a locked down system, the danger is in people using Linux because it costs nothing rather than because it gives its users freedom. Whether you agree with RMS's stance on freedom or not, it is pretty clear that making Steam on Linux a major selling point of the platform would detract from the message of freedom.
I would also remind you that the choices we hold so dear as Linux users are a direct result of these freedoms, such as the freedom to change the software you run as you see fit. Linux itself, and the GNU tools that run on top of it, will always remain free. But Linux is also a tool for promoting the idea of freedom, and Steam on Linux might weaken that.
(Personally I can't wait for it, though.)