"If I see a large group of divers clearly unequipped/unprepared to enter an inland cave I normally dive, I'd ask if they are cave trained (although it is obvious they are not), and if they say NO, I'd ask if they have the permission of the land owner to access the cave, and if they say NO, I'd ask them if they have a permit/license from the authorities (some caves require it), and if they say NO, I'd tell them that they cannot lawfully dive the cave, and if they proceed with their intentions I call 112 (the U.S. 911 equivalent) because they are clearly a public danger (a danger to themselves and others) and the police has a duty to intervene."
I can see the reasoning here, in the context of the greater situation, mainly if you are concerned these people may interfere with lines other people are running into the cave, jeopardizing others besides themselves.
But many people wouldn't get involved, and there are reasons why that is.
1.) Many people have enough issues of their own without looking for other peoples' troubles to get into.
2.) Taking it upon yourself to interrogate strangers when you are just Joe-blow average citizen with no police powers can be awkward.
3.) There's a very real chance you're going to seriously annoy someone. If any of these people know you, they could make trouble for you. Your dog turns up dead when you're not around, etc... I think of the stories we hear about horrendous neighbors making others lives miserable as an example of how starting trouble with people can come back to bite you.
4.) Even if you somehow manage to ask intrusive personal questions to total strangers on the spot in a remote location where they may outnumber you, for example, why would they choose to answer you? It seems likely they'd lie - sure we've got landowner permission! If you somehow intimidate them into leaving they may well be back shortly when you're not there.
Richard.