Aren't most autothrottles 'clamped' above a certain speed if engaged in T/O mode (actuators freeze, A/T reverts to armed for mode change after T/O)? A failure in the A/T logic would not lead to power changes in that case.
Hands are off the throttles above V1 to prevent PF from making inadvertent power changes, especially retarding them in case of engine failure above V1.