Excellent post, James. The truth is, as you say, somewhere in the middle, but much closer to the "b+!%~" side of the equation. Of course, Martin is about the worst example around, given his smash success with his unfinished series. The blatant truth is, the man is set whether he writes another word, and any more from him is only going to be icing on the cake. For almost all other professional writers, you're right.
However, I think there is another part to the unspoken contract. Put simply: The readers have the right to expect that the next book in the series will conform to the expectations of genre, style, continuity, quality and author that the first one established. You really shouldn't write one book, then start its sequel by drastic shifts in genre or setting. You shouldn't start ignoring your craft (writing as well as you can) just because you got a contract for a series. You need to be the one writing it, it's not enough to have a ghost writer. And... both in your style and your continuity, you need to make a decent effort to cleave to what you set up in the first book. Of course, the tolerance of a certain reader to each of these things varies, and some depend on the brands involved, but a radical break at any of these points is something that will have people say "I liked the first one, but then in #2 it just wasn't any interesting and I couldn't read it." As you say, promises the writer made that give returns as an investment by the reader.