That's it, exactly. It's an affront to them, to their "normalcy." And they're not going to change that to please anyone they deem less than normal (vegetarians/vegans.)
Plus, I do think some of the anger is justified. We all know a vegan or vegetarian that went overboard in preaching the benefits of eschewing meat. That attitude has hurt those of us that take the more diplomatic route, the one where you can discuss cooking and eating any food with an omni without automatic judgment. It doesn't mean that we approve, only that we aren't going to tell them that they're wrong, and horrible, and whatever else the proselytizers spout. If an omni wants to learn about vegetarianism, then they will, and more likely accepting it if they do so on their own.
Now, I did notice that when people in that thread asked for recipes, and I supplied them, that none either thanked me for them or acknowledged it. Oil well. If even one person gets some benefit from them, then that's good
One thing I've noticed with where I live (Houston) is that while I have been attacked by hard-core carnivores for my choice, that other people (usually coworkers) have attacked them for being jerks. I usually dismiss the jerks and hope they'll forget about it eventually, and that works for the most part.
Another thing your reply has reminded me to do was to send off an email to a place in Austin called Jack Allen's. I was there a couple of weeks ago with family, but as it's a grill, it wasn't a place I'd normally even look at twice for eating vegetarian. My email was a bit of a complaint in that while they did offer two vegetarian entrées, the one I chose still needs some work on execution. I'll let y'all know if I get a reply from anyone there. Good quality food, just not really thought out all that well as a meal.