Some fighters have a higher tolerance for taking shots, akin to Nick Diaz. They can get dropped or be put in a terrible position, but can find a way to get out of it without losing consciousness. And then there are others who either never had the ability or lost said ability, like Chuck Liddell late in his career. Or, perhaps to use a more recent fighter, Frank Mir. He's a good example to use in such a topic because even though the fight was stopped after he hit the ground, he didn't take any more unnecessary shots after getting dropped. Go back to the fight with Shane Carwin and you remember him flat on his stomach as Carwin threw punch after punch with almost no sign of stopping.
Yes, they knew what they were getting themselves into. But the referee's responsibility is to make sure the fighters don't take any unnecessary damage after getting dropped. A fighter's history is usually a decent precursor to follow when assessing said fighter when he/she is in a bad position to potentially take more damage.