Why isn't it legal to convert “pointer to pointer to non-const” to a “pointer to pointer to const”

Question

It is legal to convert a pointer-to-non-const to a pointer-to-const.

Then why isn\'t it legal to convert a pointer to pointer to non-const to a

Answer 1

From the standard:

const char c = 'c';
char* pc;
const char** pcc = &pc;   // not allowed
*pcc = &c;
*pc = 'C';                // would allow to modify a const object