My guess is that something like this is the right explanation:
In a native biotope, a species has evolved along side its prey. Over thousands of years, the chameleons whose genetics imbued them with an avoidance behaviour to certain prey items that, e.g. had such-and-such color/shape, survived to pass on those genes. Those that didn’t have these genes didn’t survive to pass on these genes, and so we end up with a population of chameleons that have an avoidance behaviour for certain prey items. This is why wild chameleons don’t need someone there to protect them from eating toxic bugs: they don’t eat them because they’ve evolved not to prey on those bugs.
But, when we put them into a non-native environment containing prey items with totally different “toxic” cues, they are—for lack of a better term—babes in the woods.