That reasoning is like saying "animals have sex out in the open, so I'm giving up sex because doing it in the bedroom is unnatural."
Personally, I think it's just as stupid to say everyone who becomes a vegetarian because they don't want to eat once-living animals is a dumbass, as it is to be a crazy vegetarian who says everyone who eats animals is bad and evil. Some people just would rather not do that, but won't be stupid and force others to do the same. It is natural to eat other living things, but just as well I think people have, for better or worse, advanced enough that what's natural need not necessarily apply to them.
Edit: Late edit of the post to make the first setence read better. Hopefully.