It's a deliberate choice by the staff, not a random omission: By not allowing the player to move difficulty up, it ensures that the player can't just lower the difficulty to get by one chapter and then move it back up in the next.
Now, whether or not that's valid is a completely different argument. Personally, I wouldn't care. It doesn't alienate me, I just wouldn't use it.
Now, whether or not that's valid is a completely different argument. Personally, I wouldn't care. It doesn't alienate me, I just wouldn't use it.