^^ it "needs" an R2 head on top because it is an R2 unit. It just uses it's spherical body to move instead of legs with wheels.
Like I said earlier, there's nothing "incredible" about the droid. The head stays on top because it was designed to do so. The ball rolls forward and the head glides backwards on the surface of the ball the same distance (or the head spins the ball or whatever). It doesn't need wires to transmit commands from the head to the body/ball.
Truth is often stranger than fiction, now THIS is weird: