There's more to emissions than just CO2...
First a historical note that it was the US that originally led when it came to emissions legislations. Lead free petrol, catalytic converters, fleet fuel economy standards, etc. all came from either US-Federal or Californian regulatory requirements. Now there are specific exceptions to that general statement (e.g. I think the Soviet Union was the first to introduce lead free petrol in the late 1940s, though don't quote me on that), but it was US requirements that really drove broad adoption of those technologies, especially in the 1970s and 1980s.
Over time, the EU also increased its standards and if you compare it to the US today, they are broadly similar. See http://www.dupontelastomers.com/autofocus/a1/af1.asp?article=regulations for more info.
Now as to your specific question about the Exige... For a car to be sold in the US, it has to be "certified". A big part of that certification is testing the engine for emissions and reliability. It's a very expensive process. I don't believe the Exige's original engine was certified in the US, so Lotus had a choice: (1) Retain the original engine, but incur large certification costs; (2) Substitute an already certified engine (the VVTI) and incur development/integration costs. They chose option (2).
As regards the Crown Vics, sure they don't have the most efficient engines. But they're appealing to police, taxi drivers, etc. because their total lifetime ownership costs are excellent -- they're large, strong (body on frame construction) -- hence good for ramming, very reliable (that unstressed engine helps) and cheap. As regards their fuel economy, they're about the same as other "full sized" sedans such as a BMW 7-series. But if your petrol is cheap, you don't care so much about its fuel economy (and hence CO2 output).
Diesels have not sold much in the US mainly because of the following:
(1) They have difficulty passing emissions tests (particulates in particular) in some states such as California and New York.
(2) Until recently, Diesel in the US has had a high sulpher content and that was bad news to the emissions control equipment.
The good news is that low sulpher Diesel is now becoming widely available and cleaner Diesel engines (such as the MB BlueTEC) are emerging. So the technical/regulatory hurdles are removed. However, Diesel has a long way to go for market acceptance in the US because to the average American, a Diesel is a foul smelling (sulpher), extremely dirty (think smoke belching ancient MB 300D), noisy and dog-slow heap of junk. Americans simply have had no exposure to modern Diesels for the two reasons I listed above. Its going to take a while for opinions to change. But change they will...
Back to CO2... there's no question that the average CO2/Km in the EU is way ahead of the US. But in my opinion, that has far more to do with the fact that EU countries (in general) tax the living daylights out of petrol/diesel and the cars themselves -- and not the emissions standards. Clearly taxes are very effective (whether you agree with them is another matter) because of the financial motivation.
Just to give you an idea of how cheap auto taxation is in the US... I have a '91 944 S2 and I just paid its annual registration fee (i.e. equivalent to the annual road tax) here in California. It was $78. When I fill my tank with "premium" gas, it costs $4.50/us-gal or only £0.64/liter. I'm from Ireland originally and I think the annual tax for that model is about €1,100/annum (20 times greater than in California!).
In general, I believe citizens of the EU are generally more willing to accept taxation if they can see a societal benefit. US taxation history is historically more confrontational (e.g. Boston tea party) and thus US citizens are a lot more resistant to taxation in general. And when taxation does happen, a significant portion of it is at the local level (e.g. my property taxes pay for my local schools, police, etc. The next city over takes care of itself). So the high petrol/diesel taxes seen in EU are politically unacceptable to the US for all sorts of historical and cultural reasons. I'm not saying its right, but that's the way it is.
Karl.
PS: Just an FYI that the 9 least fuel efficient vehicles sold in the US all come from the EU: http://www.cars.com/go/advice/Story.jsp?section=fuel&story=mpgWorst&subject=fuelList&referer=&aff=national
PPS: I live in San Jose d its on the short list to site the new Tesla factory!