This is due to FCC regulations.
The FCC has set exposure limits on how much RF energy is allowed to be absorbed by your body. The transmit power must not exceed the SAR limits. The amount of allowed power will depend on variables such as the transmit frequency, proximity of the antenna, etc.
Typically when a headset is plugged into the radio, the radio is then worn on the belt using the integral belt clip. This places the antenna right up against your body. The FCC mandates that we must therefore lower the transmit power in order to stay within allowable exposure limits.
If you need absolute maximum transmit power, you will need to unplug the headset.