Treebeard has proven too gentle-hearted a jailor after all, releasing Saruman at last from the ruins of his dead hopes. His reasoning is logical, if over-trusting of a multiple traitor.
And when, further up the road north, the travelling company overtake the Wizard and his Worm, the two wanderers are pitied - not that Saruman has anything but derision for mercy and kindness.
A beggar must be grateful, if a thief returns him even a morsel of his own.
In a very hobbit-like act of simple generosity, Merry gives Saruman all his remaining pipeweed, only to be accused of thievery and then not even having the pouch returned. But this is the degree to which Saruman has withered.