A Buddy Bike is really a serious bike,
specifically built for individuals who would otherwise not be able to enjoy riding a bike.
As can be seen, the person on the left (when facing forward) is in control of the bike, with steering, shifting, and braking. The "buddy" on the right is able to participate in pedaling, and receives the joy of riding, i.e. wind in the face, full view or where the team is heading, etc. A tandem can't really capture such an experience, and the coordination and teamwork required to effectively ride a tandem is most like beyond the capabilities of the "buddy."
I hope this makes sense to our C&Vers. I realize the unusual design provokes our vintage and "normal" riding sensibilities; however, the Buddy Bike was specifically designed for a specific group of individuals. The team which did so should be commended for their efforts and their results.