The sub-pixels are physically oriented horizontally normally; if you flip your monitor, they become vertically oriented, and most OSs/font renderers aren't set up to do sub-pixel antialiasing in the vertical direction, so they just disable it. It might not be much of a win to do it, either, since it'd just be giving you more effective resolution along the height of letters, which isn't usually the direction most in need of it.
Nonetheless, I think that's a small price to pay for being able to fit a vertical PDF on a monitor nicely.
