Is it a problem to connect a controlled current source in parallel to a voltage source?
As you would expect with a voltage and a current source in parallel, the current is defined by the current source, and voltage is defined by the voltage source.
The RF choke therefore constrains the DC value of the drain voltage to be equal to the supply. The FET causes its average current to flow through the RF choke.
It's at AC where things get interesting. The RF choke now behaves like a current source. We now have two current sources in parallel. The load resistance controls the voltage that the current produces.
At AC, the voltage can swing above and below the supply voltage. If the load had been a resistor, then the FET could have only pulled the voltage below the supply.