> What's the author's justification? What's your justification?

I’m neither of them, but chances are that’s because you can’t make them left-to-right all the time.

  double const *foo; // foo is a pointer to a const double
  double *const foo; // foo is a const pointer to a double

compile and do what the comment says; these do not compile:

  * const double foo; // a pointer to a const double named “foo”
  foo * const double; // foo is a pointer to a const double

I use the "right-to-left" style myself. To me, the qualifier (in this case, const), applies to the item to the right. This could be confusing:

    const char *const ptr;

The first const applies to the char, but the second one to the pointer itself. Being consistent:

    char const *const ptr;

The first const applies to the item to its left---char. The second const applies to the item to its left---the pointer. To recap:

    char *ptr1; // modifiable pointer to modifiable data
    char const *ptr2; // modifiable pointer to const data
    char *const ptr3; // const pointer to modifiable data
    char const *const ptr4; // const pointer to const data

Readability.

C declarations can become unfriendly by being too complex and disordered.

Nothing seems wrong with “volatile double pointer as a constant” or “constant character pointer” either, tbh. The way you presented is equivalent, but non-idiomatic, people would stumble upon it often. To become more readable universally this must have been adopted 50 years ago.