The pen for Wacom tablets is powered from the tablet itself. This is how pressure detection and buttons can work on the pen.
The Apple patent states:
"Additionally, the sense circuitry 704 can be connected to a power source 706, such as a battery, built in the stylus. In another embodiment, power can be supplied from a power source in another electronic device, such as a touch sensing device, via a cable connecting the stylus to the device, or via inductive coupling.".