I do not feel Specific acted very well in this situation as he never explained or even spoke to you, however, the
"MOTD" does not say anywhere regarding "Illegally parking". You were, in this situation, allowed to "Car jack" the vehicle because as seen in the rules;
3.2 Car Jacking - Players must not 'car jack'. You may lockpick an unoccupied vehicle, but if there is an occupant you may not steal it for thieving purposes.
However, why is it you felt the car was illegally parked? It was parked on one side of the road, allowing a clear space for other cars to pass. Would you not feel, seeing as you were a "Police" job, that messaging Specific and ask him to possibly move the car before warranting and taking the car?
I feel Specific did not do a great job at handling this situation, however strongly disagree he should be warned for it. At the end of the day, you took a car which did not belong to you without reason.