1 It’s how it was designed, the inputs are normaled this way.
2 There is no way to check if nothing is plugged in.
3 So, no way to automatically zero the unused inputs.
This probably could have been done differently.
Edit:
I knew I’d read about this before. On Rev6 or newer Patch, the normaled inputs can be disabled by removing some resistors. Still a bit simplistic, I would have preferred non-normaled inputs be grounded when no cable is present. And even then, reliable cable detection in software wouldn’t be possible.