Board Thread:Questions and Answers/@comment-24499616-20140208032904/@comment-8986886-20140209012815

It could be that Hell can't access Souls, once they're in Soul Society, and by bringing Ginjo, Tsukishima, and Giriko to SS, the Shinigami were able to protect them from Hell's influence; despite their past atrocities.

The Fullbringers also used Hollow powers to commit their crimes, and a Zanpakuto can purify a Soul of any crimes made while being a Hollow. Perhaps the Fullbringers' Hollow powers created some kind of loophole, allowing them "absolution" of their crimes as well?

Whatever the case, it wouldn't surprise me if Kubo had something else entirely in mind though. ^^