Blame the general public for that one. Anyone with half a brain can work out that a car that emits 100 carbons per mile and does 10k miles a year is more polluting than a car that emits 200 carbons but does 3k a year, but that's not good enough for the masses.
They want to see anyone with anything better than they've got punished, despite the fact that we're already getting punished when we fill up (not a complaint, it's our choice and we know this) as our cars use more fuel to go faster. These masses wouldn't stand for a fairer system, and any government who tried it would find themselves out on their ears very quickly: Just witness how unpopular the coalition is, despite the fact that the opposition contributed massively towards the current mess and would have had to do pretty much the same things themselves had they won the last election.
People are idiots, and short-sighted idiots at that. Anyway, rant over, off you go folks.