I like to think of it as a shared risk; the author takes a massive risk in writing something (for FREE) that may never be read by many people, so I think it’s only fair the reader take the risk in reading something they didn’t pay for that may never be finished.
The thing is, though, that if the reader comments on the work it could bolster and encourage the writer and make it more likely the fic WILL be completed, so. Circle of life, and all that.