If I had to guess the rationale (stupid as it is), they might want to make sure anyone who “self-deports” doesn’t come back. Or the detainee was wanted for something else.

While looking for articles about this, a Laotian guy under a 15-year old order to leave tried to get permission from Laos and couldn’t. He was detained during a yearly check in. ICE said they didn’t believe he had any intention of leaving. They haven’t deported him yet because (drumroll) they haven’t gotten the travel documents from Laos.
https://www.cbsnews.com/colorado/news/keooudone-phetchamphone-ice-laos-self-deport-colorado/