They are probably built for it?
This WOE/WOL is not necessarily the end all, be all for everyone. It works for a lot of people, but just like everything else, it is totally different for each person. Genetics and where you are descendent from are probably two factors.
I know a couple vegan, natural (no steroid/HGH/etc) body builders. While they aren't massive, hulking figures, they are pretty damned big guys and they compete (and place) against other guys that eat like we do.
People too weak to follow their own dreams will always try to discourage others.