In the 1960s, Harley-Davidson was struggling. Honda, with its line of popular scooters, was not. So Harley-Davidson tried to break into this market starting in 1960. The "Topper", powered by a 165cc two-stroke engine, was marketed as a recreational and utility vehicle, useful for fun excursions and package delivery. It even came with a sidecar option for people or cargo. The line never took off, though, and Harley-Davidson ended the experiment in 1965.
Link | Images: Modern Vespa and Motorcycle Info
My other thought for the design stems from their Hummer.
Harley Davidson came out with the Hummer. It's design was based on the DKW Rt125. After the war, the designs, tooling equipment and some of the staff from the DKW plant ended up at the MMZ factory in Moscow. So maybe that is the connection?
Harley was not original in their designs of the Topper or Hummer. They just copied what was popular.
I used to have one back in the 80's. It was fun to tool around on, but I always drove it on side streets. Had I know they were popular today, I would have kept it.
But I am currently happy riding around on my DKW rt250.
Or was american design really that square and pukeadelic as the pictures show?