User blog comment:Xilinoc/Ch. 639 - BABY,HOLD YOUR HAND 2/@comment-2211792-20150820080309/@comment-11051098-20150821183703

That's an interesting theory. Possessing information about the Soul King sounds like a reason serious enough as to why he was the only prisoner held separated and in chains.

It doesn't quite explain why they actually decided to release him though, when he accepted Urahara's invite to join the planned SRDI. If he was locked up for possessing information about the SK, was there any possibility that he would ever become less dangerous?