Lots of monitors eat plants, in fact, in one book, Goanna Book, they use a pie chart and plant matter is listed.
On occasion, we offer my sons tortoise food to many species of monitors, and again on occasion, they eat it. Some fairly regularly, some not.
Also, I see many species of insect eating lizards, eating plants. Of course the books do not say they do, but they do. On another forum, I give day to day updates on a wild Scaly lizard, I have for four years now.(during the summer) and every year, she decides to eat cactus fruit. She gets that telltale, cactus juice face. I believe I posted pics of that. I have also seen them eat flowers.
The question is, why do people believe books. Or at least take what they say as rule. I get the feeling, biologist, have a huge need to classify things, and so they stick species in catagories. I guess if you were to put monitors in a catagory, it would be carnivorous, because they mostly eat meat(animals/insects) but surely the books do not stop them from eating plant matter too. No matter what their skull and teeth are shaped like. Kinda the same for cats and dogs(wolves and wildcats) they also eat plant matter when they feel the need.
Unlike Shvar, I think they eat plants when they have a dietary need to do so. It could mean, something is missing in their normal diet. It could also mean, your not feeding enough. Or there could be no reason at all. Please consider, young monitors in the right temps, are bottomless pits, they are starving within a couple of hours of feeding. While us keepers think we are feeding enough, signs like eating plants, the water bowl, anything it could get its mouth on, are signs that we are not feeding enough. Good luck, FR