What special relationship?

I've mentioned before that I live close to the M6 Toll Road. I hardly imagine that US Embassy officials would venture this far north of Watford, but if they did, and availed themselves of the Toll Road, I would expect them to pay the toll - indeed if they didn't they would be blocked from getting on, or getting off as the case may be. America has many toll roads, and Britains, including embassy officials, have to pay to use them. So why does America (and others) insist that the London congestion charge is not a toll (which it obviously is), but a tax, and that therefore under diplomatic rules, they are exempt from paying it? Apparently, they currently owe us to the tune of some £3.5 million. Pretty shabby way of treating a supposedly friendly country, don't you think?