Fast Ebike

I saw this article about a Fast Ebike and it got me wondering about the upper limits here.  At what point does a fast ebike become a motorcycle.  I rode a vintage electric bike and it was fast.  Going 28 miles an hour seems really fast.  I am not sure I can convince myself the $4000 price tag is worth going ten miles an hour faster than my current ebike.  It would same me some time on my commute.  I am planning for my current ebike to last me for years to come.  When I need to replace my ebike, maybe I'll get a fast one.