OK, here's my two cents after finally getting to watch the return over and over.
Williams had his right hand cupping the nose of the football when his knee hit the ground, so he should have been ruled down. It is difficult to maintain that grip on the ball if the ball is loose. However, because the back of the ball cannot be seen, it could be a sliver of doubt that he had full control over it.
I could see the officials overturning that call, though, because of Williams' hand placement on the nose, but can understand to some degree why they didn't.
If he had been ruled down on the play, there is no doubt the call would have stood.
That's probably the primary problem with instant replay: There are times the officials are willing to let the play continue instead of blowing it dead because they know they can look at it on replay. But replay gives the benefit of the doubt to the call on the field instead of what the initial ruling should have been. If the officials had seen the play clearly they wouldn't have called a fumble because the evidence didn't support it.