From reading http://theory.stanford.edu/~rvg/arroyo/ (warning: nudity) linked from the ropewiki page, when hiking from the top, this journey is a combination of hiking, wading along the river and floating down it. It sounds like lots of fun. Turning around at the waterfall means that they would have to hike back as well as swim upstream, possibly through rapids, to get back to their starting point. It could be next to impossible.
I could see this being an easy trap to be caught out by if you had limited exposure to conditions in the area, or even if you had checked weather conditions and missed a rainfall upstream while out hiking. While they might be ashamed of needing a rescue, they thankfully made a smart choice and this is a cool story rather than a search for bodies.
Just went to that waterfall in July. Even when water isn't flowing too fast that rope is hidden behind the water... you just have to pull it out. Another option is just ride the water down... 3 small waterfalls that end up in a deep pool so not too dangerous. My friend Scott did it without rope just for fun. He did get a bruised leg but otherwise no injuries.
This wasn't the most robust crew. They were at a local minimum, both topographically and risk. They were safe where they were for at least a few days. Hiking out might have been beyond their ability. Having read some stories of people who didnt get out of these situations, it's not usually the first mistake that does it. It's the process of trying to get out that results in digging a bigger hole. I think they made the right call to abort rather than increase the risk.
Because descent is usually trickier than ascent. If you've ever climbed down a ladder missing a rung there is that brief moment of free fall where you assume where you're going to land. But if you're on a rain slick, moss covered trail a controlled descent can be difficult even under ideal circumstances.
I could see this being an easy trap to be caught out by if you had limited exposure to conditions in the area, or even if you had checked weather conditions and missed a rainfall upstream while out hiking. While they might be ashamed of needing a rescue, they thankfully made a smart choice and this is a cool story rather than a search for bodies.