This enclosure is basically an island just its missing the water......what it dearly needs is planting and lots of it and possible a deep layer of straw on the floor indoors. But this is one of the better ape enclosures there.
I think its some sort of cultural/historical edict that maybe the old ladies had- mow the grass short so the that the public can see the animals properly? But I agree in this day and age it seems very unnecessary and makes unnatural enclosures look even more unnatural. They have quite recently added all the climbing equipment around the dead tree which was originally the only thing in here. More for the Bonobos to use but more vegetation and uncut grass would make it far more interesting visually(and for the apes).