If all the games respected HID and Valve did something proprietary, I would understand the skepticism. The truth is that most games are engineered with platform integration (e.g. for achievements, controller mapping, etc.), and fallback to the Xbox API. It's reasonable for Valve to sell a controller that takes full advantage of their platform.

Also, Valve's primary OS is Linux-based. There's surely either already a module upstream in the kernel or one is coming soon. That is: open source software to take full advantage of this controller. That's not the same thing as a walled garden.