Torres, a CASA audit or surveillance can only pick up what they see on the day, or are shown in the records. For example, in my most recent job, I was one of only two Captains on the fleet with a current RNAV approach approval. None of the F/Os had it. That did not preclude us from flying 'practice' RNAV approaches in VMC. But when the wx was IMC, to stay legal we had to fly whatever approach both of us were approved for - usually NDB or ILS at our particular destinations.
But suppose we had illegally done a RNAV approach but shown it in the aircraft trip logs and our own logbooks as NDB. How could CASA know unless they were there on the day and witnessed it or another operator knew and dobbed the crew in?
CASA certainly had a part to play in the Lockhart disaster, but on the day the only persons in a position to prevent what happened were the two pilots. It has been established that the unfortunate F/O was not properly trained for what they attempted, so he is far less culpable than the PIC but nevertheless he obviously was there for the ride and we must presume either agreed it was a reasonable thing to be doing - or was not able to object, perhaps for fear of his job. CASA can't dictate RNAV endorsements for all pilots unless RNAV is mandatory for a particular operation. Any more than they can stop pilots flying under the influence of drugs, unless they test every pilot before every flight.
Other major factors were the approach plate design and aircraft equipment - CASA again. But the guy who drilled it into the hill was ultimately responsible and we as a pilot community have to accept that, as unpleasant as it may be for the deceased pilot's family.