Exactly. I believe it is a great way to sample other specialty classes before sinking money into them à la carte. Certain skills, such as navigation, night diving, and search and recovery, could easily be pegged by a diver without the need for formal training. But it's a helluva time to find out what exactly you don't know and aren't prepared for when you are 50' under water with zero experience and supervision in the new task or environment. But having the instructor to mentor the diver and prepare them for expected problems while helping them work through the unexpected problems that stem from lack of experience, can make the difference between going on to enjoy the type of diving or not, or even getting hurt.
Charters want to ensure their clients are able to do what they went out there to do. If they are planning on hitting dive sites that are all at depths greater than what OW divers can do, and that's all there is to do, then they will require it. Often, but not always, dive sites will have stuff to do at shallower depths and things to do at deeper depths. But if all there is at the site is deep diving, they ensure that the divers on the boat will at least have been certified to those depths.
Yes liability is involved. Insurance policy rules and regulations dictate a lot of what a commercial operator will offer. As an example you have Joe the Divemaster. Joe is leading a couple of tourist divers on a dive who rented some tanks and weghts from a local shop. At the dive site one of the divers has a problem with a piece of his personal equipment, say a fin strap. Joe can step in an provide a spare strap, or even a spare set of fins. But if Joe doesn't also have an equipment rider on his professional liability insurance, Joe wouldn't be covered if that diver gets hurt as a result of the fin strap failing that he provided. So if Joe doesn't have said rider on his insurance, he may not provide the fin strap. Currently that's a $433/year add-on to his $575/year DAN liability insurance (and a fair bit more to a $1331/year Vicencia and Buckley policy, another large professional scuba liability insurer).
It comes down to having someone there to guide you through the process. Sure OW Sal can drop down to 90' and look at the pretty fishes. You Googled all you need to know. But what happens when at 90' narcosis kicks in and Sal goes completely dumbfounded while his buddy swims off and Sal's air gauge dwindles? Or Sal completely panics at 80' and bolts for the surface because he wants to orally inflate his BCD at depth to save air and doesn't put his regulator in his mouth, despite having it there in his hand? At least having been in a supervised training enviornement an instructor can recognize and address the problems Sal may have before they manifest and spiral out of control into a panic situation that can get Sal hurt or killed. Or worse, someone else hurt or killed.
If there was a standards violation, by all means report it. Sure nobody likes to get called out for something they did wrong. But if they continue with such a pattern of behavior it creates a
normalization of deviance to which problems ultimately arise that never would have presented should proper procedure have been followed.
PADI, NAUI, et al, have and will investigate or otherwise take action on such claims of standards violations. Their findings may not become public knowledge, but they may (and have) take more subtle actions such as