Paging Mr. Occam
His reasoning? Well, everyone would naturally think that Putin was involved in the assassination of one of his harshest critics, which means he couldn't actually have been responsible. Does that mean O.J. is innocent by default, too?
The other possibility is that Putin has gotten away with brutally silencing dissent before and has no reason to believe he won't continue to get away with it, what with a lap-dog press and an international community worried, above all else, about instability in Russia.