Rumor is he did that b/c Scarlett Johannsen didn't want to duet with him.
Not rumor, truth! From his blog (because I'm kind of Files and Records like that sometimes):
I hear Scarlett Johansson's putting out a debut album of Tom Waits covers. I'm a fan of Waits so I started pestering her management around a year or two ago to duet with her. She rejected me like I was in high school again with an alleged "Oh no, I hate Fall Out Boy." Fair enough, that's not the first time I've heard that this week.
Nonetheless, I was bummed because I really just wanted to sing some Tom Waits. So I sat at a piano, pressed record, and plunked this one out. Were I trying to impress message boards with my knowledge of Waits, I certainly would have picked something more obscure. Perhaps something off the out of print Night On Earth soundtrack. Or the song he wrote for Solomon Burke. Or that early demo of his I think is called "Up Shit Creek," or something to that effect. But fuck that, the value of scene points is dropping like that of American currency or my patience for LA drivers. I just really like this song, so I did it."