Do you want an iPad/iPhone/WinRT/Android tablet with "apps", or do you want an x86 device with the ability to run whatever binary code you want and an OS that enables that.
It's not about price, because you're right, I can find devices in both categories at literally all sorts of data points. But it is about the experience.
Valve's business RELIES on the experience that enables users to run, in this case, native code. Since they're being pushed out as a result of this shift... they're naturally trying to warn people and shift interest to a platform where they can more easily operate... or survive.
In fact, your example is ironic. People are more likely to go buy a $500 iPad than they are a $300 full featured Windows laptop. That's exactly what worries Valve. Or it should.
Amazon sells a Mac Mini for 600EUR (at least Amazon.fr does). That's about what an unsubsidized iPhone costs, right?
So where is this doom-and-gloom everything-is-locked-down-except-the-ultra-expensive actually coming from?