pulse1, I don't see why a helmeted pilot of a multi-seat aircraft would be duty bound to provide similar headgear for all passengers. Helmets are very expensive, and (IMHO) to work well they have to be custom made, or at least fitted with a semi-custom liner.
It would be nice if everyone could have a helmet, but I wouldn't refrain from wearing mine just because a pax had none: my greater safety doesn't adversely affect his (now parachutes, that's another matter).