I'm glad that the powers-that-be consider it a motorcycle. If they considered it a car (defined for the purposes of this thread as four or more wheels), then all the federal safety standards kick in. Air bags, crumple zones, seat belts, etc.
if it's a motorcycle, that keeps the machine much simpler, and retains the fun factor. IMO, the Slingshot and Elio (upright seats and a steering wheel, instead of straddling the machine and using handlebars) are pushing the envelope, and may eventually draw the unwanted attention of the full regulatory state.
As a sidenote, BMW once looked at an open-air four-wheel toy, the "
Just 4/2," using one of its motorcycle engines to move it. It remained a concept only, as (from what I heard) the corporate lawyers weren't about to let it enter production as it was, since it lacked a lot of the mandatory auto safety features. As an engineering exercise, it was fun, and a lot of folks drooled over it, but it was not to be. Once you built it up to safety specs, it would cost as much as a car anyway.
Thus, classifying three-wheelers as motorcycles, regardless of layout, is an escape clause for fun.