It seems to be coming down to the Texas Department Of Public Safety clinging to the definition of a motorcycle as something with "a saddle" as opposed to a car-style seat, which the Slingshot has. As such, the vehicle is no longer street legal in the Lone Star State.
I think they take the same view in Australia. Morgan, or the Oz importer, has spent a fortune getting compliance. In Australia it is considered to be a car.