He had it in his hand. If he could pick it up what stops him from just taking it back? Then if he had written Kal Snyder in it he would have been holding it, wouldn't ownership go back to him like it would if another human is holing it when the owner dies?
shareSidoh was a pushover. However, there was probably a rule that the shinigami can't take back the book if the current owner is still alive and in "possession" of it.
shareAlways wondered how strictly the rules needed to be followed. We only know that if a Shinigami kills to deliberately lengthen a human's life that Shinigami will die. Ryuk kept saying he was forbidden from telling Light names, though would that rule or stealing a notebook back result in turning to sand as well?
If he had stolen it back at that point Mello would probably freak out, the notebook flew, wrote a name in itself and Kal Snyder died.