Nick
what you report is true only partly in my hopinion but is indeed intriguing and interesting to think of.
Although I find myself shooting VERTICAL most of the times ( I often take close portraits of people ) I realize two things:
1) Yes, the head is NOT tilted when looking at things around us but is also true that our eyes have a much more horizontal scan of the things than vertical. I think I've read somewhere that both eyes cover a 120° from left to right but for sure they don't do the same vertically (at least without moving the chin)
2) Despite my tendency to shoot vertical, I think it's more related to how close or how surrounded you want a subject in the place it is. If you have to frame a human body AND you want to be close, you MUST frame it vertically, unless you have a very wide angle with you. Conversely, if it's NOT the simple people but the whole scene to be interesting, then the horizontal framing lets your eyes free to scan around it and discover maybe something new or interesting. In general, the same picture taken with different perspective may be very different: try to figure three old men/women framed alone and vertically. One may think those are special or famous men/women. Try to figure them within the same (horizontal) photograph where aside them there's a young rock concert which first was not viewable in the vertical framing. At this point men's value in the image changes a lot. They are no longer important because of them, rather because they maybe show the "curiosity" of the old age toward something young. First it was a "dull" image, now you have a strong contrast which is the real subject of the same image.
I know I'm not yet that expert to shoot as old and famous reporters (pjs) but for sure this is a way I'd really love to. As someone said, "it's easier to shoot with a 90 mm than with a 24/35 mm " Language and what is expressable are really different there.