I had a feeling I was getting my impressions of the Bosmer from biased sources, since I've not played through their zones or in their alliance in ESO. Thanks for setting me straight, with good-humored roleplay to boot.[Dúathil extends a "dinner invitation" to the human with a menacing smile]
Hey, you're lucky we Bosmer are considered to be the only elves with a sense of humor!
Jokes aside, even if my parents were apostates, let me shed some light on your prejudice:
When the Mer first came to Tamriel, my Bosmer siblings were the ones who ventured forth and made their home in the lush and impenetrable jungle of the Valenwood. The forest god Y'ffre assumed patronage of my ancestors and gave them the permission and abilities to thrive in this otherwise hostile environment in return for the oath known as the Green Pact.
My people lost their shape-shifting abilities and can't take, eat, kill or otherwise consume and harm any vegetation of the Valenwood, as it's the sacred dominion of Y'ffre the Forest God.
Greenspeakers can form and grow architecture from living trees without harming them. Otherwise, shelter, tools, weapons, music instruments etc are made from bones and hides and the like, and skulls are a popular decoration. My people are hunters, especially skilled archers, and use every little piece of the killed animals.
To never kill wastefully and never let anything to rot is part of of the Green Pact and the main reason for what you call "cannibalism". The Bosmer of the Valenwood honour their own dead by consuming their flesh, and that of their fallen enemies. They don't hunt people of sentient races for sport or as a delicacy. But think a bit, with all the alliances and wars you're talking about: Would they have happened if you valued every kill enough to consume the whole flesh of your opponent?
With that said, I don't know everything. I'm an orphan and my parents were apostates who had left the Valenwood to live in the Great Forest in Cyrodiil, because my mother loved alchemy and my father loved her. I only know what they told me until I lost them, and what I've read in books.
Besides, the Green Pact only applies to the vegetation of the Valenwood. The use of imported wood for example is allowed, and many Bosmer who live in other parts of Tamriel, where any vegetation can be used, don't even follow the Meat Mandate any longer. If they do, they will only eat your flesh after they have killed you for a good reason. Never fear, they won't lure you to their camp to bind you to a cooking spit .