Because they eat mainly or exclusively plants and not vegetables.

Could it be that vegetarians are making a social/political choice rather than following their natural drives? Humans like many animals are omnivores right? Bears are assumed to be vicious predators but they eat fish, carrion, berries, insects and processed food products humans bring into their environment. Are bears making a social/political choice in their diet? Calling people vegetarians? Is a a human conceit? Something to make them feel better about themselves when it isn't...well I won't argue it's merits that is another question.

Sometimes they are called vegetarians.

Because calling an animal a herbivore is the way that they were made, a natural instinct to eat plants not animals. Calling someone a vegetarian is describing their state of mind, that even though they could eat animals they chose not to. That is the difference between a herbivore and a vegetarian.

Also it is not a choice for herbivores, it was evolution that decided there stomach acid formula's & teeth structure. to be a vegetarian is to have a choice between eating vegetables & not.

Herbivores also don't have a choice on what they eat, they physically can't digest meat. Whereas vegetarians choose not to eat meat, even though they can.

POETICALLY: We do call some animals herbivorous Instead vegetarian souls And we appreciate them when they eat flesh Carnivorous or omnivorous; not barbarous!

Vegeterians can change anytime. Animals can't do that. A horse will starve in a butcher shop, just as a lion will starve in a field of wheat.

The term herbivore, and its related terms carnivore and omnivore, come from biology, and are used to express obligate lifestyles of animals. The term vegetarian comes from human society, and is used to express a social and cultural choice. Human are omnivores, but can choose to be vegetarian. I see vegetarian as an expression of choice, and applicable only to humans with an ethical sense.

A herbivore is an animal that maintains a plant-based diet, carnivores maintain an animal based diet and omnivores consume both plants and animals. The fact that humans have certain traits and characteristics that resulted from an omniverous diet or that our ancestors evolved as omnivores is irrelevant to the fact that a human who abstains from animal products is a herbivore. Not all of us are omnivores and we aren't "hardwired" to consume flesh. Our ancestors were opportunists. Somebody said almost all animals are omnivores, this is false. Animals that are normally herbivorous will only eat insects or other animals when facing starvation. Furthermore, just because an animal can consume animal protein doesn't make it an omnivore (all mammals, at some point in their life, consume animal protein from milk. Cows in slaughterhouses are given animal foods but we know that they are adapted for a herbivorous diet). Controversially, one could argue that humans are actually adapted for a herbivorous diet, especially considering that we are the only 'omnivores' who suffer from hypertension, high cholesterol, atherosclerosis etc.

Because vegetarian is a term for omnivore humans who choose a diet without killed animals. 1) "The Vegetarian Society, founded in 1847, claims to have "created the word vegetarian from the Latin 'vegetus' meaning 'lively' (which is how these early vegetarians claimed their diet made them feel) ..." However, the Oxford English Dictionary and other standard dictionaries state that the word was formed from the term "vegetable" and the suffix "-arian". The Oxford English Dictionary also gives evidence that the word was already in use before the foundation of the Vegetarian Society: 1839 - "If I had had to be my own cook, I should inevitably become a vegetarian." (F. A. Kemble, Jrnl. Residence on Georgian Plantation (1863) 251) 1842 - "To tell a healthy vegetarian that his diet is very uncongenial with the wants of his nature." (Healthian, Apr. 34) But notes that "The general use of the word appears to have been largely due to the formation of the Vegetarian Society at Ramsgate in 1847." " Source and further information: http://en.wikipedia.org/wiki/Vegetarian#Etymology 2) "We cannot blame ourselves for immoral behavior just because we eat animal flesh. We evolved as omnivores. We can't help our evolutionary past. But we did not evolve as cold blooded carnivore killers like sharks or crocodiles, either. We must teach our young to kill animals. Our ability to hunt and kill animals does not come from instinct but rather from social conditioning. And this gives us reason to modify and evolve our ethical behavior without contradicting our animal nature." Source and further information: http://nobeliefs.com/comments9.htm

Because they might like fruits.

"Also it is not a choice for herbivores" Of course it is. Human herbivores choose to avoid animal flesh for ethical, environmental or health reasons. Non-human herbivores avoid animal flesh because they simply aren't hardwired to view it as food. Both human and non-human herbivores choose not to eat flesh. Cows don't consciously realize that they've been adapted for a herbivorous diet, it's just what they consider to be food. Humans are animals. It's laughable that we think otherwise. There isn't some magical, dividing line between ourselves and the rest of the animal kingdom.

Because we are allowed to eat herbivores, but not vegetarians.

Herbivores eat plants. We should be calling Vegetarians; Herbicides. "Plant Killers"