"Why are meat eaters so angry about the beyond meat burger?"
The only reason why "meat eater" are getting so angry about the beyond meat burger, is that they see it as catering to a small minority, and don't believe that a minority should be catered to. I've seen it happen with those who aren't afflicted with say celiacs, or has an egg or dairy sensitivity issue. I've seen some of those who get angry with that, who has stated that as such there is no such thing, and that it's all in the mind of the person.
Personally I could care less about what another choses to neat or not eat, or their reasoning for doing so. Not everyone has the same dietary tastes, or even follows the same diet. However if a person likes the Beyond Meat products that's fine, it's nothing worth my effort to get angry about.I have enough things that annoys me, without adding more to the list. However some people seem to have a deep need to get angry with anything that doesn't follow what they chosen to believe, and believe that everyone has to believe exactly as they do, otherwise they feel they're rightfully entitled to be angry at those who fail to accept what they've decided to believe, and expect everyone else to do the same, and obey their dictums.