As I understand it, this is due to what actually happens when you "push" your development.
In reality, pushing does almost nothing to increase the detail in the shadows. What it does is increase the contrast across the board. As a result, the appearance of the deep shadows doesn't change but the lighter areas (from fully detailed near shadows, through the mid-tones and up to the near highlights) becomes more contrasty, and therefore either more or as appealing. The brighter highlights, however, lose detail due to the increased contrast, so they suffer.
Kodak doesn't recommend the one stop push because their results indicate that the loss of quality in the highlights outweighs the benefits obtained in the near shadows.
If you have a scene with a narrow subject brightness range, a push is much more likely to work well - but that is probably true even if you don't under-expose the shot.
So essentially I'm saying that if you have a scene with a narrow subject brightness range, go ahead and meter your scene at EI 800 when using ISO 400 film and request a one stop push. If, however, you have a scene with an extremely wide subject brightness range, you need to decide where you are willing to sacrifice detail - in the shadows, or the highlights.
If your scene has a moderate subject brightness range, you can rely on Portra's excellent ability to capture it.