I think that a lot of people sit back in powder out of the natural instinct to brace yourself against the "unknown." And I suspect too that many skiers are uncomfortable when they can't see their ski tips, so they sit back to bring them to the surface.
But you do NOT need to sit back to keep the skis from diving. With your weight pressing down in the middle, and the snow pushing up evenly along the base, the skis will bend into an even arc that will keep them floating in the powder if you keep your speed up. Standing balanced over the sweet spot is as important in powder as on the groomed. You do want to keep some pressure on the tips, as well as the tails.
On the other hand, the POSITION that results from being balanced over the sweet spot WILL be a little farther back in powder than in/on "faster" conditions. Remember that the position of balance differs depending on a number of things. Even the type of wax on your skis will affect they way you stand on them for balance. Powder is "slower" than most other conditions, so balance requires being a little farther back. This is NOT the same as "sitting back on your tails"!
Furthermore, in "bottomless" powder, your skis actually float IN the stuff, rather than gliding ON it. In the snow, they often float a little "tips up"--not at the same angle as the slope. So your balanced position on them will LOOK a little back to an observer. They can't see your skis--they only see your body position relative to the snow surface, and it looks "back" even if you're balanced.
So there are several reasons why many skiers actually DO sit back in powder. And more reasons why even balanced skiers may give the impression that they are sitting back. The myth of "sitting back in powder" will probably live on for a long time!