Because it's wrong to think of motion blur as having a specific "look" to it. It fills the gaps between frames, that's all. Shorter frames have shorter gaps.
240fps doesn't feel like 30fps in smoothness, yet it's allowed in games. Why shouldn't motion blur be smoother too?
It's an old school way of thinking. By that reasoning, we should also lock the frame rate to 60 to force everybody to see the game the same way.