I suggest that Timothy is correct about Washington State.
It would seem that my source (an owner of a 2012 M3W) may have creatively understood what he read in the RCW (revised code of Washington). (OK...I'd rather look something up than argue over beers....no sense of fun.)
http://apps.leg.wa.gov/rcw/default.aspx?cite=46.37.530RCW 46.37.530
It is unlawful...
(c) For any person to operate or ride upon a motorcycle, motor-driven cycle, or moped on a state highway, county road, or city street unless wearing upon his or her head a motorcycle helmet except when the vehicle is an antique motor-driven cycle or when the vehicle is equipped with all of the following:
(i) Steering wheel;
(ii) Seat belts that conform to standards prescribed under 49 C.F.R. Part 571; and
(iii) Partially or completely enclosed seating area for the driver and passenger that is certified by the manufacturer as meeting the standards prescribed under 49 C.F.R. Sec. 571.216.
I hardly expect that Morgan has made any such certification as noted in iii. Yet, the Polaris Slingshot with same or less protection seems to be driven here without helmets. In practice, I don't know how much emphasis is put on the certification itself, or whether it is simply that people are in a partially enclosed seating area.
49 C.F.R. Sec. 571.216 -- appears to be a federal vehicle safety standard dealing with roofs that withstand rollovers. Doubtful that the M3W meets this standard nor has been certified.
https://www.law.cornell.edu/cfr/text/49/571.216I suppose it partially comes down to enforcement:
Are you willing to gamble that you can either
- talk your way out a stop (the vehicle has a steering wheel, seatbelts and partially enclosed seating area with rollover protection)
- or just get a warning.
Hm.