Because Marines do not trust pirates, Shichibukai or not.

Eventually Moria would have an army too big for the Navy to glance past. They aren't risking it.

4 points I think should be made.

1. The Guards torture inmates to death daily.

2. In level 1 to 5 daily death rate is at least 7 for floor.

3. World government could use this agreement to control morgia by threaten to killing few inmates. If he disobey their orders or rebel against them. Their release someone from random levels every 1hour.

4. Random things like a dragon want new slaves or want inmate's df or illness.