"We can predict perfectly what [Rater C's and Rater D's] next rating will be, regardless of the quality of Phish’s next performance, so they are of no help in assessing show quality."

This would be true if C and D rated literally every show. But they don't. I'm obviously just speculating, but my guess is that Rater C only rates shows they think are under-rated, and D only rates shows they think are over-rated. If I'm right, their votes could actually help assess show quality.

Note that I am not condoning such rating behavior. I think people should rate a broad swath of shows and give their honest assessment, independent of what the current show avg-rating is. I'm just pointing out that the entropy concept is potentially misleading in this context. Ratings are complicated!