Why can't I turn off the zero client right after it is powered on?

The power system of vCloudPoint zero clients like S100 is controlled by the kernel (the residing software).

This allows users to power off the device through software interface (e.g. the login screen) without needing to press the switch button on the device, which is impossible with those powered-by-direct-hardware (button) devices. This brings convenience when the devices are back mounted at the monitors.