Because "That is the compiler's job: emit the code that the programmer wrote", and optimizing means the compiler is allowed to emit code the programmer did not write.
> If you can optimise without compromising the intended semantics of the code, go ahead.
And how does the compiler know what the "intended semantics" of the code are, if it isn't precisely what the programmer wrote?
> you will have to apply judgement
How is that compatible with "It is not the compiler's job to second guess the programmer and generate the code that it believes the programmer meant to write"? Applying judgement to guess what the programmer intended sounds exactly like "generating the code that [the compiler] believes the programmer meant to write".
> Why bother with optimizations, then?
Why not?
If you can optimise without compromising the intended semantics of the code, go ahead. If you cannot, do not.
Note: you will have to apply judgement, because the C standard in particular happens to be incomplete.