The extra gas causes a rich condition which when unburned would release more hydrocarbons than it would when burned. Remember in a perfect combustion reaction; Hydrocarbon + Oxygen = CO2 + water, of course it's never perfect, which is why we have emission standards and testing. Unburned, vaporized octane is going to increase hydrocarbon output which makes the car dirtier emissions wise, and the simplest solution car makers can do is just to burn it off.
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