Pre HST the tax payable on a private vehicle sales was PST (Provincial Sales Tax) but not GST (Goods and Services Tax). When those two taxes were combined to form the Harmonized Sales Tax, the Automobile Dealers Association lobbied the provincial government very aggressively to increase private vehicle tax to the same rate as HST. They portrayed themselves to be at great disadvantange having to "add" 13% to the selling price of a vehicle, whereas a private seller only added 8%.
OpenGambit is correct, this is strictly a provincial tax on private vehicle sales that happens to match the GST rate. The provincial government had the option of actually reducing tax on a number of items under the HST deal with the feds, but chose not to, and instead added retail tax to a number of items (mostly so called sin items) to match the HST percentage, and to appease lobbying factions.
The UVIP actually will show a $ value for a motorcyle, but it is currently not used to determine the tax due.
