Why Doesn't the Foucault Pendulum's Oscillation Plane Rotate with Earth?

[cepheid]

Can somebody explain to me why it doesn't matter that whatever structure from which a Foucault Pendulum is suspended is also rotating along with the Earth? How come the pendulum's plane of oscillation is not altered?

Wikipedia sort of has an answer, but to me it seems to amount to little more than a claim that it doesn't matter:

No forces act to make the plane of oscillation of the pendulum rotate - the plane contains the plumb line, so the force acting on the pendulum is parallel to the plane of oscillation at all times.

 

[Vanadium 50]

The pendulum's support can't put a very large torque on the swinging pendulum system, as the lever arm is almost zero.

 

[russ_watters]

Note that the ball will rotate. It's just the plane of motion that doesn't rotate.