... Why do singers think it's okay to cover other singers songs, and basically ruin the song?

So, there I was, minding my own business… Just listening to Cherry Bomb by The Runaways with my sister, LezboPirate. Then, suddenly, we came across Miley Cyrus completely destroying a cover of the song. By the end I heard Erikah (LezboPirate) go ”.... Oh my god.” in a disgusted whisper.
Why do people think this is okay? I assure you, it’s not. I am offended.