The pin, although labeled "Bias" (which has special meaning in linear amplifier design) actually provides the only path for DC voltage into the device.
For years I've seen this arrangement of applying voltage and I'm not sure if there's a performance advantage to doing it though the RF output pin.
Clearly, it has the potential for reducing pin count (and there are plenty of three-terminal amplifiers out there), but for the device in question above, there are unused pins.