I was under the impression (based on Hard Knocks & other info that came out at the time) that Hamilton thought the ball carrier had stepped out of bounds & didn't want to get penalized for a late hit. Not that he quit on the play or made a business decision not to tackle him in a lost cause.
Still a mistake, but big difference in what it says about Hamilton as a player and what can be expected of him going forward. He seemed pretty distraught about it on HK or wherever the footage was that I saw shortly after the game.
Doesn't seem at all like Hamilton to deliberately let the guy run by him, from what we know about him. But then... who knows, I guess.