Ok let me alleiviate some concerns
Ok look. I get that heavy metal has a relatively bad rap in modern society. This is because of it being ignored by myriad common people who can be satisfied by modern popular music like Ke$ha and Justin Bieber. But metal is important and valued to some people. And that should be respected. My point in this explanation is to point out that the "Not for kids" label is a bit extreme for its content, and the reviewer docked points left and right for any reason they could think up. They say the word "cigarette," yet any child can walk down the street in multiple places that are within 10 miles from their house and see lots of cigarettes, alcohol, and other "inappropriate content for children." The other issue I have is the blatant cheap shot and male teens claiming that Slipknot only resonates with male teens. I may be a male teen, but there are plenty of females who enjoy metal of all occasions. Additionally, it says to talk to your children about who would listen to this kind of music. The phrasing of this statement makes it seem pretty discriminatory against people who resonate with this type of music, which is pretty unfair, because I could write an entire book about why kids shouldn't listen to modern pop music. Lastly, all teens are angry. I agree that children under the age of --say-- 13 doesn't need to be listening to music this intense, but just because a teen is out of control doesn't mean that Slipknot made them that way. Children are a product of their parents, so limitations and childish rules (like controlling your teen's life with remedial, conservative websites like this one) would definitely make your kid upset. So, with that in place, let your teens listen to metal. It's really not an issue.