A vegan diet is a subset of a vegetarian diet is a subset of an omnivorous diet. It's taboo to feed a vegan meat because it lies outside their dietary restrictions, and it's not taboo to feed an omnivore plants because they're within their dietary restrictions.
More importantly there's the moral implications of it. Vegans are (generally) morally opposed to eating meat, but omnivores are not morally opposed to eating plants. Imagine a greater set of foods of which an omnivorous diet is a subset. Say, human veal. Imagine you lived in a society where the majority of people were okay with eating human children, and one of them tried to fool you into eating one. That would be an entirely different situation to you trying to feed one of them turkey, right?