reason is that some applications have hardcoded coordinates/layout in it and they are screwed on that orientation. If you really want to use reverse orientations you can just look component gallery sources on gitorious (you can download the example from developer mode menu).

Its not a secret, but its disabled so that all applications would work. And its really stupid usability wise anyway.

guillermorojaz: you only asked if its possible or not, you didn't ask how to on original question. So the question deserved yes/no answer.