Oberon, whatever the legalities that have been arrived at by the powers-that-be it has to be commonsense that Police Observers are crew, in any logical use of the word.
They need to be there in order to conduct the task since the Pilot can't do the stuff that you do. If the observers are not there the aircraft would not fly.
There is a more complex agenda to all this, of course, to do with licencing, regulation, training and COST.