The dog jaw and teeth are also that of a carnivore. The teeth are large and pointy and made for ripping meat apart. The early dogs jaw is very powerful and on a fixed hinge that provides very little lateral movement so that they can lock onto prey. Animals made to eat greens must generally crush them into a pulp to assist with extracting nutrients from them. Omnivores have large flat teeth for grinding and shredding the greens that dogs do not have.
Dogs have high acid bellys to quickly digest meat, and very short intestines to expel the potentially dangerous waste as they often eat rotting meat that would kill a grass eater.
The evidence that I hear most people cite about why their dog is an veggie eater is that their dog will eat all the left over on their masters plate, or that they feed it a whole array of fresh veggies on purpose. Others tell the story of how their dog regularly eats fruit off their trees. Readers should be aware that one of the primary reasons that dogs fit into our lifestyle so well is that they are a pack animal and generally born to please us. If you have fed your dog garbage from an early age, you might also think that because it continues to eat it, that this is the best food for it.
The counter argument is that a dog fed on a healthy meat diet will usually not recognise any veggies or bread as food. They will however invariably enjoy dog pellets and many canned dog foods, not because they are nutritionally balanced, but as they are full of jellies and sugars and artificial flavours that are attractive to their taste, just like humans often get addicted to junk food..