Impossible to overtake safely - so no, the following drivers were not at 'fault'. If an inconsiderate driver is holding up half a mile of traffic, he/she should pull over.
I don't disagree that slow drivers should pull over when there is an opportunity to do so to let traffic past. I would contend that a road in which it is impossible to overtake safely anywhere is probably not capable of supporting traffic moving at 50-60mph.
I suspect what might have been actually happening here was an example of what is to my mind the worst form of driving, where someone drives very slowly though bendy bits, but then speeds up when the road straightens out, preventing people from overtaking.
I have no problem whatsoever with people driving slowly, there may be a million reasons why it's the right speed for them, but this 25mph through the bends/50mph on the straights nonsense is extremely frustrating.