It was logical. Gollum would inevitably go search for his Precious (though it wasn't truly his) and he would lead its true owner straight to it. It was unfortunate that there was something about him that refused to be broken, but the information already gained would be enough.
There was a nagging little sense that told Sauron he had just done something extremely foolish; that there was another reason he had let Gollum go.
But Sauron resolutely refused to admit that the reason he had let that miserable creature go was because he saw Gollum as a reflection of himself.
