Because they won a “metal” Grammy. A lot of people got super pissed off about this, because the category they won is called hard rock / metal.
To explain: Dear metalheads, hard rock / metal means hard rock OR metal. Not hard rock and metal.
Either way, blame the people at Grammy’s who lumped bands like Dream Theater, Mastodon, Megadeth with Sum 41 and Foo Fighters. Dave Grohl did rock out a Slayer shirt (as seen above) so that makes it completely okay, right?
The thing I only care about is finding out if Skrillex won the best singer Grammy <3