Big John stopped the fight at exactly the right time. Chuck legs had gone dead, and, while he wasn't completely unconscious, he was defenseless.
I've said this before, with MMA blowing up, the last thing the sport needs is a serious injury or death because a ref didn't get in there on time.
In boxing, when a fighter goes down, the other fighter, unless he's deranged, will step away and ref will begin the count. If the fighter doesn't get up by 10, that's it. There's no decision for the ref. In MMA, you're allowed to hit a downed fighter, so, the ref ALWAYS has a decision to make. Some are easier than others. That's why you'll always have more second guessing of refs in MMA.