Treebeard releases Saruman.
Not knowing that Saruman still had plenty of power left in him Treebrard released him from Isengard with the intention that this fallen wizard would go into exile. Fans of the movies are unfamiliar with this storyline and at this point in the books the company leaving Rohan had no clue this happened either. Saruman was, of course, defeated but by no means was all his power gone. While the strength to flee was still in his veins he had a few more vicious strings to pull elsewhere.
Treebeard’s intentions were pure, but a little naive, we got that feeling from Ents when we first met them in The Two Towers. They aren’t fully aware of the dangers of the world around them, therefore they don’t have a full understanding of the effect their decisions have on other beings in the world. That’s just our rational for why Treebeard made this decision, beyond the obvious, Saruman looked defeated angle.
Thank you for your time and remember, from the ashes a fire shall be woken.
– The Wandering One