>Drivers rating the passenger is a defence against passengers rating the drivers low

I don't see how this it the case, assuming that drivers don't know what passengers rated them before rating the passenger.

Reciprocal bad ratings from several drivers could indicate a bad passenger who is being fussy etc and trying to take it out on drivers.

But as I say in the GP, I am assuming that the drivers don't know that they are given a low rating prior to giving the passenger a rating.

It's not a low rating that triggers a reciprocal low rating. But an obnoxious and fussy person who gives out low ratings should be discounted if in turn multiple drivers have given him/her low ratings (for being obnoxious).