I've put way too much thought into this and it seems apparent to me that the prison barge is simply a freighter of sorts which has been converted to prison duty. It's best for the operational fleet to isolate the prisoners who can't be reformed immediately and to do it safely. Niven teaches us that all spacecraft are weapons, so keep the number of vulnerable ships to a minimum. Additionally, it's a great plot device.

Irony: ammo says it's time. Tom is blocked.
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