I wonder if the issue for the US was not wanting any publicity about her husband's identity, since he was working there as some sort of spy.There are a lot of accidents that occur on UK roads due to the non-UK driver forgetting and driving on the right instead of the left. The woman didn't kill this young man intentionally and she seemed willing to help police in the enquiry. Clearly, the State Department didn't want that.
That, and American entitlement, of course.
The strategy backfired and now they've got a front page international story instead of a back page local one.