Wouldn't it be the same thing as a regular
meat-eater visiting a friend and finding out they're having freshly cooked
dog, and then calling them rude when they don't
partake of your
carefully and
painfully crafted feast?
Maybe the problem is that it's not rude for the vegetarian to refuse to eat meat, but that it's rude to serve meat to someone who is a vegetarian? That you consider your guest(s) when preparing a meal, instead of expecting them to conform to your tastes? If you know the person has special dietary concerns, whether by choice or not, you should make every effort to accomodate them. If you don't know, then ask them beforehand. Even if you fail to properly accomodate them, whether or not you'll try and do so speaks volumes about you. And if somehow someone shows up who has special dietary concerns, but you were unable to prepare for them for some reason (for example, they came with someone else who neglected to mention the special diet), then you should not interpret them refusing part of the meal as rudeness.
Or if you can't go without something in a meal, at least prepare an alternative, or at least a meal they can avoid the meat if they so desire? So beef stew or chili with meat would be bad choices.
Regardless, it requires the participation of two people for a host/hostess to be offended by a guest refusing part of the meal.