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I know your view is adopted by Encyclopedia Metallum (assuming you know that site) and, surprise surprise, I completely disagree with their decisions on what to exclude from their databases.
My view doesn't align with Metallum, I absolutely hate that site. They don't allow nu metal, funk metal, rap metal, most industrial metal, a handful of 70's metal, or pre-Sabbath metal. They also include many bands that I don't really think are metal.
The way I see it, is that metal is completely incompatible with pop music which Amaranthe borrows heavily from. Since its inception, metal has been a complete opposite from pop. With what I consider to be one of the first metal songs, it has this unique heaviness, darkness, and energy that screams heavy metal. It's a raw burst of emotion and distortion, and evokes feelings that pop music (at least from what I've heard) isn't allowed to express. There's no connection to the 60's pop of the time: