Hmmmm, thats a head scratcher, if it was only about generating revenue, why limit the number of vehicles? Which it turn limits revenue. There could be safty issues as well as emission issues that come into play. Certainly revenue has been mentioned, by the State, as a leading factor, but I think it's more than that.
Hawaii also limits the number of Kit vehicle registrations per year, based on safty issues, not emissions or revenue. During public hearings on the "new law" concerns were expressed that allowing to many potentially high horse power vehicles (Cobras were mentioned) or potentially unsafe vehicles (Dune buggies were mentioned) per year might be unsafe for the public well being. You could all but hear them whisper, "What about the children!"
The island of Oahu is therefore limited to 100 per year, with no limits on the other islands.