That's what puzzled me: if British TV can handle it in a way that the TVs latch onto automatically (it's something in the decoding, as they do look different from set to set, and particularly if you record a DVD copy on one machine and play it on another), why not do the same in the states?
It's not about the TVs. It's a specific choice made by the people behind this specific set of specials. I've never seen it done with anything else in American TV. Usually, programs are broadcast in their original aspect ratio, or portions are cut off to fit the screen but the actual proportions of the image are unaltered. (Although a lot of HDTVs do stretch things out to fit the frame unless you set them to display the original aspect ratio. But that's done by the receivers, not the broadcasters.) "The Aztecs" was presented in its original aspect ratio when this retrospective began. But starting with "Tomb of the Cybermen," they decided to employ this bizarre distortion that kept the central part of the image in its original proportions but stretched out the sides of the image to fill a widescreen frame. That's something new in my experience and I profoundly hope it doesn't catch on.