It does that when you move the root bone up in IK. But when you move the root bone down or back, the human will squat or sit.
- Squat
- Sit
Now, if you move the root bone up only a little, the human will sort of stand on their toes...
- Tiptoes
But if you move the root bone up too far on the Z axis, you get distortion...
- Distortion
This is the nature of reverse foot rigs, which are good for some applications and not so good for others.
I have found that IK is better for posing humans for still shots, but that FK is better for animation, especially mocap. Of course, it all comes down to personal preference, so your mileage may vary, as they say.
The cool thing about the switchable rig is that you can choose which one works best for whatever you're doing at any given moment.