That's about as far as Obama is prepared to go so far. He has just stated that the US is sure the aircraft was downed by a SAM from rebel-held territory. They are not confirming who fired, from which system, or why, at the moment.
He is all but saying that the separatists shot down MH17, but is refusing to confirm such.
Why? He must be waiting for more absolute evidence, perhaps copies of the tapes from air/ground/space intelligent sources. Or the evidence comes from something too secret to be reviled.
Then again, knowing President Obama, who knows?