FWIW, the safety card and video clearly indicate that the shoulder seat belt only needs to be worn for takeoff and landing. It is meant to be removed during flight.
This may be the first time you've seen it, but such shoulder straps will become much more common in coming years as regulators have stepped up head impact safety rules -- many business class seats will either need to use a shoulder strap or an airbag integrated into the seatbelt, as some already do.