Of course it is music. Even Techno is music despite the fact it typically does not contain live instrumentation, it still has rhythms, themes, lyrics etc.
Music is a form of entertainment.

Something as technical as Heavy Metal is definitely music, some of the best music is arguably made by Metal bands. Whether you like a genre or not does not make it 'Not Music'. Metal has some of the most technical time signatures in songs. Progressive rock/alternative bands are known for technical time signatures and crafting music that is not cliched.

Despite the simplicity of pop music, and the cheesy lyrics, it is still music.

If you are not well versed in the music scene, do not even discuss such things, it makes you looks silly. I do not mean to sound harsh, but seriously, your opinion does not determine what is and is not music. Music is music by the definition of being music, despite different tastes people may have.

Metal is music. It's all music. What these people don't understand is that all music represents an emotion. Dance music is for people who like to dance. Pop is usually for people who want to feel good etc. Metal, is an expression of disgust with society... rage... rebellion and a feeling of "we're not taking this shit laying down anymore.. fuck it... lets FIGHT".

It's as valid as any other form of expression. It's just that people don't understand the expression of anger.

It can be more than entertainment. Some times, music can be a political/social tool... meant to create or provoke change and or thought. Not all of it is meant to be background noise while you clean your bedroom etc.

To me it's just painful expressions of angst, pain, unhappiness, frustration, etc. I don't really enjoy it.

But I know others that do. I'm more particular than I think most on here about my music - I like them a bit lyrical and with a melody - the kind some hate.

So I'd put Metal as entertainment/art. Just not one I particularly enjoy that much.

It's all music by definition.

Yes, they are music. Even though I don`t listen to screamo, I'm not going to go around claimining that it's not music. I love metal, and I enjoy a lot of electronic/atmospheric music (even if it's not techno).

I may not like this music, but I can't not call this music.

However, I'll call your taste shit and happily go along on my way. :^)
It's okay, they'll call my taste shit too, so it's all even.

It's all music, but why is Metal being lumped in with the rest of that garbage?

Unless your idea of metal begins and ends with crap like Linkin Park, Limp Bizkit and Slipknot, 2 out of 3 of which aren't even real metal and the other is borderline.

They're bad, but they're still music anyway.

I listen to bands like Entombed, Opeth, Meshuggah, Sepultura, Death, Atheist, Dismember, Cannibal Corpse, Darkthrone, Emperor, Burzum, Carcass, Deicide, Dissection, Metallica, Megadeth, Slayer, etc. Yet Linkin Park is still my favourite band, and I really do enjoy Limp Bizkit and Slipknot. As for whether or not they are metal, well there is no standardized regulation and classification so it all depends on the individual.