It all has to do how they are shown on the SD broadcast (remember, they comprise the largest audience). For Leno, NBC centercuts the 16:9 and crops out the center to 4:3; hence, everything is framed for 4:3 and anything on the sides is "extra space". For Conan, his show is letterboxed on the SD broadcast, so they frame for 16:9. Some widescreen movie clips are upconverted and shown full-width, though there have been times when Conan gets HD clips. For Leno, even the letterboxed clips are pillarboxed to make it easier for a 4:3 SD centercut; to us HD viewers, we see black bars all around!
Yep - there is also the issue that promotional clips are often released in letterbox standard def 4:3.
You have a choice of either zooming the clip full frame - which will look REALLY soft in HD (it is bad enough in 16:9 SD) - or pillarboxing the 4:3 image containing the letterbox, and ending up with postage stamp/windowbox. The resulting image is smaller, but looks sharper.
If your programme is centre-cut cropped to 4:3 full-height for analogue platforms then you could end up with a soft AND cropped image if you zoom full-frame in 16:9 HD. If your programme is letterboxed on analogue platforms it wouldn't look so bad on analogue I guess.
I suspect sourcing HD film clips is quite involved - though Leno and Conan are presumably high-enough profile shows that studios will be more likely to treat them as special cases... It bodes well for the future if Conan is able to source HD clips somehow.
Some EPKs (Electronic Press Kits - I film clips, interviews and B-roll) seem to be appearing in the UK in 16:9 standard def these days (presumably the B-roll is shot 16:9 for DVDs?) - though I'm still not sure they are the norm. Just as well really - as all network broadcasters are 16:9 these days.
(Same with pop promos - many of these are still delivered 4:3 - I guess because almost all of the music video channels are still 4:3 in the UK)
It is a real issue if you are trying to avoid postage-stamp / letterbox images in 16:9 - which can be a "technical transmission review" fail point for shows delivered to the BBC...
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