I suppose Hatori has no choice. When Shishou was describing the bonds it sounded like something entirely out of their control. He is incapable of leaving while the spirits still possess them.

And the thing with Kureno, I really think that he stayed with Akito because of the screwed up bond too. Basically, yeah, he is physically released from the curse, but I think the psychological dependency is still there and would take some time to dissipate (only Akito won't let him). It's so engrained in him by that point to bow to Akito's wishes and to subserve himself all that, that even though technically he doesn't have to any more, psychological dependency is even harder to break and doesn't just turn off with a switch.

Gosh, I hate Akito.