Quite apart from the stupidity of wearing such an expensive necklace at night in an entertainment area, if indeed that was the case, I would have expected the clasp on a 100,000 baht necklace to be suitably robust, which would make it difficult to forcibly remove without the wearer being aware of it at the time.
As an absolute minimum, the insurance companies must surely be requesting proof of purchase with a receipt. Perhaps, in future, just as they do in the UK when refusing a pay-out in a household burglary when the owner has left his door unlocked, here they will start to refuse claims for jewellery insufficiently protected when stolen, by either being hidden, or heavily clasped or under a certain value.