Up until now, I thought what he did was grounds for disqualification or at least against the spirit of the rules.
But now I have rethought it. I think what he did makes perfect sense (but may still be against the spirit of the rules or etiquette of the game. This is a different question). And at first I thought his claim that he had thought about doing it before, in other situations was b.s., but now I think he was probably telling the truth.
Let's simplify the math. Let's say he was putting for birdie. He putts it by the hole and seeing it is headed off the green down the hill, he moves very fast and when it is 1 foot past the hole and still moving, taps it in for par+2 strokes penalty, a double bogey 6. If he had let the ball go and declared it unplayable, he would have had to 2 putt from the same spot to make a 6. Not sure how hard a 2 putt would have been from where he was, but in some spots, like above the hole on #18 on Saturday, a 2 putt was very difficult indeed. His other option, to let the ball roll down to where it would have been would mean he would have to get down in 3 from that spot to make the same 6. A player could very well judge that hitting the moving ball was his best option--certainly the simplest and quickest one.
The USGA needs to change that rule as soon as possible. Hitting a moving ball intentionally should never be an option.