People are tossing the words "cavern" and "cave" around as if they are interchangeable. They are not.
The fact is that whatever the manual may say, OW divers take guided tours of caverns every day around the world. It would not surprise me to learn that 1,000 OW divers participate in a supervised cavern tour every day around the world. It is done routinely in many places, and OW divers often seek out those places for their reputations. In many places these caverns are routinely dived--with permission--by OW divers without guides.
It is my understanding that this was supposed to be just such a cavern dive. (People who know the site please correct me if I am wrong.)
Once they were inside the cavern, the DM apparently mistakenly took a wrong turn because of his unfamiliarity with the site and because a usual warning sign was not there for some reason. In doing so, he took these divers into a cave. Not only was it not a cavern, it was particularly dangerous because of the presence of a great amount of silt. Divers who are diving under the belief that they are part of a routine cavern tour will have no way of knowing that the professional guide they are following has led them out of the cavern zone and into a particularly dangerous cave. Without the warning sign, how were they to know?