Here's my guess as to why it would work like that. Random software package either supports the EXIF orientation or it doesn't. If it doesn't, Joe User just sees the photo rotated wrong, and they want to fix it. Joe knows he can rotate photos in Windows Picture Viewer. So he opens the file in Windows Picture Viewer and it ignores the EXIF orientation too, so he just hits rotate then goes about his business. If Windows Picture Viewer respected the EXIF orientation, it would be much less useful for this task.
Given that, OS X Preview has a different approach. It respects the EXIF orientation when you open a file, but when you re-save a file, it silently rotates the file and resets the orientation. Not sure if that is better or worse.
Given that, OS X Preview has a different approach. It respects the EXIF orientation when you open a file, but when you re-save a file, it silently rotates the file and resets the orientation. Not sure if that is better or worse.