Right, which is why I was saying that some ethical theories can't be expressed by a utility function.

Ah, I see. But I'm still not actually sure that's true, though...see below.

I'm surprised at that +b, because that doesn't preserve utility ratios.

Indeed not; utilities are measured on an interval scale, not a ratio scale. There's no "absolute zero". (I believe Eliezer made a youthful mistake along these lines, IIRC.) This expresses the fact that utility functions are just (scaled) preference orderings.