Decieved? Persuaded rather. He may have had that, but if there was no pity at all to be found in Fangorn then he would have kept him locked up. Saruman's persuasion (in this example) is used to find that pity and bring it to surface. If there was no pity to find, what good does your argument make? Perhaps so, but what did Saruman do then? Took over the Shire. Now honestly, do you think that the greatest of the Istari (a very elite member of the Maiar) could be considered anything but on a downward spiral when he's reduced to controlling the pipe weed import/export of the Land of Halflings? Persuasion isn't a skill you can loose... it's something that once you know how to do it, it stays with you. As long as he doesnt damage his voice in any way... Did Treebeard know a thing about Gollum? Could Treebeard know what it's like to be betrayed? You can't blame him one bit for thinking Saruman was reconciled. Who knows? Maybe Treebeard knew that Saruman couldn't do much damage besides the damage he'd do in his inferior state. Even if he didnt, he was still correct to release Saruman. He can't be entirely dumbfounded and blind to what Saruman still has left.