When I participated in an anthology to benefit 9/11 families, the pieces that weren't *quite right* for the book were supposed to be posted on the website. I don't think it ever actually was done, but it was a nice idea, because as Jenna mentioned many pieces weren't rejected due to quality but for "fit."
I don't think pieces that don't fit the theme should be posted on the website because people may go there to see what might be in the book, and then might not buy the book thinking that the pieces on the website were intended to reflect the theme of the anthology!
As we knew so well, but are seeing played out for real now, writing is just one tiny part of what makes a book, and an even smaller part of what makes a book sell. Jenna, I wonder whether anyone whose work didn't get put into the book, but whose efforts help to publicise it could have a mention of thanks on the website?