Sure, I have to use my gaming console as a gaming console, much like I use my smart thermostat as a thermostat and don't check email on it.
And all you use your IPhone for is to make phone-calls, right?
But in this case, they even say:
"...and it's a PC
Yes, Steam Machine is optimized for gaming, but it's still your PC. Install your own apps, or even another operating system. Who are we to tell you how to use your computer?"
It's not just a gaming console.
It's doesn't have to be non-gaming purposes. Say you want to install something not sold on Steam like I dunno, World of Warcraft, or Minecraft.