A catch is determined by two things.
If the player is not falling to the ground, he must make a (or have the ability to make) a football move while maintaining control of the ball.
If the player is falling to the ground, there is no football move requirement, but the player must maintain control of the ball through to the ground.
That catch was clearly #2. He had to maintain control through to the ground. He did not. Just because he is in the endzone doesn't magically remove those requirements.
In response to this play, the NFL has stated it was a judgement call. They did NOT say it was a sure thing. They said it was a judgement call by the ref. I think he was wrong. He was falling to the ground. He certainly didn't make a football move, as described by the rules. He was falling to the ground, which means he had to maintain control through the catch. There is no magical moment as soon as you go over the goal line that it's a touchdown. If that were the case, ANY ball that simply passes through a receivers hands, and even held onto for a moment, would be a touchdown. That's not the case. Just touching a ball with two hands while in the endzone doesn't mean an instant TD. Especially when catching a pass.
It's a huge deal because it was upheld through a review.. and had we lost, I think folks would be singing a different tune. Next time we (or another team) may not be that lucky.