Because the American Govt has never been caught lying about a major incident/accident before have they?.... They were too quick to lay blame, there is more to it than what's apparent now I reckon. But it definitely benefits Ukraine as the story is now. A bit suspect that an incident on this level is what it would have taken to shake support up to intervene or break the deadlock situation in Ukraine then what do you know, plane shot down. I'm not saying who did it either way, I'm just saying that its a great opportunity to bend narrative to suit a given agenda.
Just because the people in the "audio clip" spoke Russian don't make them Russian. A lot of Ukrainians speak Russian too.
Also why would you down a civilian plane from damn near on your own border? That doesn't help your cause? Sounds counter productive to their mission.
It's easy to say you have images of the launch when you are not required to release the images