James Hetfield declined the invitation
In an interview with Rolling Stone, vocalist and guitarist Sammy Hagar spoke about his new album, Sammy Hagar and Friends. The CD, scheduled for release on September 24th, features several special guests, including Joe Satriani, Kid Rock, Nancy Wilson (Heart), Mickey Hart (Grateful Dead), among others.
When asked which guest he would have liked to have on the album but couldn't, Hagar mentions James Hetfield. "I was going to call him for the track 'Knockout Dragout,' but since I met Kid Rock first, I asked him to record it."
Hagar then recounts that he called Hetfield to sing a smaller part of the song, and the Metallica replied: “I’m flattered that you want my voice on your album. But it’s not my style.”
Hagar says she replied, "Okay, but I still love you, James .