Heres my view on this question being a vegetarian. It was difficult for my husband particularly to bring a meat eating animal into our lives. We considered feeding vegan dry food but decided that it was not in the best interest of the dog particularly when it is a growing pup.
For us its an environmental issue. the production of chickens, cows etc for meat is not as environmentally friendly as production of vegetable matter. A dogs meat eating diet is an extra burden even if it is their natural inclination. When you feel so strongly about being vegan or vegetarian particularly for the environment it can be difficult to get around the idea of owning a meat eating animal let alone buying it meat. I guess if people feel that strongly then they really should own a rabbit. If its an ethical/environmental issue for some people then i don't see why they cant feed byproducts like chicken necks, offal, chicken frames and chicken/bone mince.
I have a question though in regards to those who feed only dry food. If you can get a regular dry food and a vegan dry food and they are both fairly equal in protein, fat, fibre etc then is there really much difference between them. I agree that people trying to feed a dog vegetarian via mixing miscellaneous vegetables together is potentially hazardous.