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 pointer to pointer to const?
E.g., why is the following code illegal:
char *s1 = 0;
const char *s2 = s1; // OK...
char *a[MAX]; // aka char **
const char **ps = a; // error!
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