My mom doesnt like me listening to death metal?

I was listening to Slipknot in the car on the way to school (they arent death metal, I know they ar nu metal), and she started saying that she doesnt like them because they are death metal. But they arent. I told her that she doesnt like their image. But, she also looked through my iPod and saw Deicide, In Flames, Children Of Bodom, Suffocation, Job for a Cowboy, Slayer (not death metal either), Suicide Silence, Norther, Amon Amarth, and Soilwork. She got mad becuase most of those bands are either death metal or melodic death metal. Why do you think she doesnt like me listening to death metal? BTW: She has no problem with Lady Gaga or Miley Cyrus (my sis's music). And they are CRAP