The high nutrition value for fruit basically frees herbivores to start sliding down the omnivore track without really "losing" any of the nutrition.
The way the Diet stat works is that herbivores get 100% of the nutrition from fruit, carnivores get 0%, omnivores 50%, and so on with all the in-betweens.
So, if you're an herbivore who is really good at finding fruit (i.e. you evolved where fruit was very rare and not very nutritious, and could only survive by evolving to actively seek out fruits) and suddenly every fruit you find is more calories than your body can even contain, you lose nothing by letting your diet ratio slip down toward omnivory, but you DO suddenly gain energy from eating other critters (say, when you bump into each other and you either bite them or let them bite you; or when you encounter a dead body just sitting there for free.)
This is something I've observed many times -- you want to see the Diet graph shift away from all yellow herbivores? Make fruit MORE nutritious, so critters don't need to invest in plant digestion to use it. Instead they can invest in meat digestion and still supplement their diet with fruit, and get a decent return.