In order to achieve the appropriate position in a high-bar squat your knees are going to be well ahead of the toes, as far as ankle flexibility permits. You need that very upright, low hip posture to get the most bang-for-your-buck when squatting for weightlifting. The hamstrings aren't creating major hip extension as they are in the low-bar squat, but they are still stretching and contracting while high-bar squatting. The emphasis is just placed much more on the quads and glutes.
Also note that on really heavy back (or front sometimes) squats, you can see weightlifters shoot their hips back for a very slight portion of the movment, but then they shove their hips back under their heels. That's really the only time the hamstrings have a major contribution to the movement.