His basic argument is that the contract is you paid $10 for each book you got, so the author doesn't owe you the next book.
I disagree with this. First off, the fact that the fans are so upset is evidence that this wasn't _their_ understanding of the contract.
The thing is, they didn't just spend $10. They spent time getting invested in the story. This emotional investment is left unfulfilled if they don't get the implicitly promised ending.
Now, let's turn this around. Let's say fans do decide that their $10 strictly comes with no expectation of the next book coming.
Well then many fans wouldn't have bought the first books until the whole series is finished. It would likely become uneconomical to write long fantasy series. Writers like Martin would suffer.
It's easy for Martin or whoever to drop the ball on the implicit contract now, after becoming rich; but if this contract hadn't existed, authors like Martin would probably have desired to create it explicitly. If you believe that, then Gaiman's argument doesn't hold up.
And so not finishing a promised series is taking from that trust commons.
Of course, sometimes humans _can't_ finish something, and readers should accept that risk. The authors shouldn't be harassed.
But wanting them to follow through is not the same as thinking "they're your bitch".