A few points here:
All the US prices you see are tax, registration and delivery exclusive. These charges vary by state, but on, say, an 'entry level' C-Class might be around $5000. I *think* the tax % rates increase on more expensive cars?
I'd tend to disagree about the spec's - there's usually a much more limited range is the US (especially engine choices) and options are usually in packages. BMW is a good example of this - the 'entry level' 3 Series is a 325. It's a very similar spec to UK SE version but includes Sun (they call it Moon) roof but doesn't include parking sensors. There's a limited range of options and option packs. Options are usually much cheaper than UK.
I reckon the main reason for reduced choices is that Americans generally won't wait for a car - they like to buy from stock, so the manufacturers have limited the engine/option choices available to make stocking them more viable. Of course the Americans are also not interested in a C180K, or a 318i, as they've only got 4 cylinders
Many new cars are private leased in the US and it seems staggeringly cheap - I have a colleague in the US who's just got his wife a Porsche Cayenne for $600/mth, and he drives an E55 at $800/mth. He told me that the standard College Grads cars are either 325 or C240 - they're the entry level 3 or C in the US and both can be had for $249/mth.
One thing that he (and others) have never been able to explain to me is why used cars are so expensive in the US. 3-4 yr old cars are about the same price as here. It kind of makes sense to have low lease rates if the residuals are high, but then if the cars are cheap to start with, why buy used?