I guess this specific cave being so off limits might have fueled the idea of picking exactly that one, they might very well have been oblivious to the damage they were doing.
That makes me think the opposite, I broke into multiple caves as a (stupid) teenager and had no intention of doing anything destructive (besides underage drinking).
I doubt they were trying to vandalise Aboriginal art, since it seems they only covered finger flutes which are hard to distinguish as art.