I think metal has far too many elements, and is too broad to stick into a corner like that. To me its about entertainment just as much as self-expression, and while the musicians should naturally feel more connected to the music, the listeners matter as well. To me a big part of metal is in the shared comradery, metalheads tend to stick together, treating it almost as a defining part of their lifestyle. Which is where I come in saying that writing a song about metal, and its followers is very acceptable in my opinion. It's easy for me to be friends with people based almost solely on their taste in music, this is not the case with many things. Its funny how you guys pigeonhole it into a "I'm right because it's what I think" bitch fight. hahaha.
Its not hip hop where the image, reputation and center of the universe "artists" are all that matter, and the fans hardly pay attention to the music beyond "does it got bass and a gud beat I can dance to?!"