Hello Duncan,
At the risk of spoiling an amusing whimsy, the ‘flaw’ in the definition used on the page that you linked to is that it is the legal definition of the word ‘motorcycle’ used in California State Law, specifically the ‘Vehicle Codes’. See here for the base document that I suspect the supposed ‘definition of a motorcycle in California’ was lifted from. If you can be bothered to browse all the word definitions you will discover that the word ‘road-roller’ is defined elsewhere, thus clearing-up Jasonb’s confusion (section 565b Jason)!
The actual construction regulations covering road vehicles in the US are the Federal Motor Vehicle Safety Standards (equivalent to the UK Road Vehicles (Construction and Use) Regulations1986) which, as the title suggests, define, along with other road vehicles, a motorcycle (that is to say: the ‘machine itself’) by the construction controls (max number of passengers, maximum width & length, etc., etc. ad nauseam).
Sorry to spoil the fun, but hopefully only for a moment or two, as I suggest that it is best to put the ‘reality’ to back of mind and carry-on with the whimsy of ‘unlikely motorcycles’.
Phil