Why? (Aside from the fact that it was that way in 3.x)I think the "Prone" condition should attract an attack of opportunity when the creature stands up like it used to.
You can shift one square with a move action without triggering an OA, but you shouldn't be able to stand up within the same square without one? Both actions seems like rather minor movement to me, so I see no realism requirement for it.
And there is certainly no game requirement for it. Having just two things triggering OAs is one of the best simplifications in 4e, compared to the multitude of things causing them in 3.x. I don't want that back.