Harley can call it a Roadster if it makes them feel better but pretty much everyone who doesn't own one (99.99999999999999999999999999% of bikers) calls them Skirtsters anyway.
Harley has had roadster type Sportsters off and on through the years but so have various other vehicle manufacturers. Though I certainly agree an r3r would be by far the sportiest of roadsters.
I read somewhere that the name "Thunderbird" was first registered and used by Triumph for their motorcycle after a designer from Triumph visited the U.S and visited the "Thunderbird Motel." Ford then asked Triumph's permission to use the name "Thunderbird" which Triumph granted as they felt there was likely to be no conflict between the car and the motorcycle sharing the name.