Why is gwen stefani singing muse?

I love Muse and i saw gwen stefani (howver you spell her name) singing muse songs like invincible and starlight and time is running out. So im just wondering like, why? whats up with that? have more singers sung muse songs? so is muse like selling them to other singers? I'm just kinda dissapoitned so if some one could set me straight here, that'd be great :)