C Standard – Why Consider Const-ness Recursively?

The C99 standard says in 6.5.16:2:

An assignment operator shall have a modifiable lvalue as its left
operand.

and in 6.3.2.1:1:

A modifiable lvalue is an lvalue that does not have array type, does
not have an incomplete type, does not have a const-qualified type, and
if it is a structure or union, does not have any member (including,
recursively, any member or element of all contained aggregates or
unions) with a const-qualified type.

Now, let's consider a non-const struct with a const field.

typedef struct S_s {
    const int _a;
} S_t;

By standard, the following code is undefined behavior (UB):

S_t s1;
S_t s2 = { ._a = 2 };
s1 = s2;

The semantic problem with this is that the enclosing entity (struct) should be considered writable (non-read-only), judging by the declared type of the entity (S_t s1), but should not be considered writable by the wording of standard (the 2 clauses on the top) because of const field _a. The Standard makes it unclear for a programmer reading the code that the assignment is actually a UB, because it's impossible to tell that w/o the definition of struct S_s ... S_t type.

Moreover, the read-only access to the field is only enforced syntactically anyway. There's no way some const fields of non-const struct are going really be placed to read-only storage. But such wording of standard outlaws the code which deliberately casts away the const qualifier of fields in accessor procedures of these fields, like so (Is it a good idea to const-qualify the fields of structure in C?):

(*)

#include <stdlib.h>
#include <stdio.h>

typedef struct S_s {
    const int _a;
} S_t;

S_t *
create_S(void) {
    return calloc(sizeof(S_t), 1);
}

void
destroy_S(S_t *s) {
    free(s);
}

const int
get_S_a(const S_t *s) {
    return s->_a;
}

void
set_S_a(S_t *s, const int a) {
    int *a_p = (int *)&s->_a;
    *a_p = a;
}

int
main(void) {
    S_t s1;
    // s1._a = 5; // Error
    set_S_a(&s1, 5); // OK
    S_t *s2 = create_S();
    // s2->_a = 8; // Error
    set_S_a(s2, 8); // OK

    printf("s1.a == %d\n", get_S_a(&s1));
    printf("s2->a == %d\n", get_S_a(s2));

    destroy_S(s2);
}

So, for some reason, for an entire struct to be read-only it's enough to declare it const

const S_t s3;

But for an entire struct to be non-read-only it's not enough to declare it w/o const.

What I think would be better, is either:

1. To constrain the creation of non-const structures with const fields, and issue a diagnostic in such a case. That would make it clear that the struct containing read-only fields is read-only itself.
2. To define the behavior in case of write to a const field belonging to a non-const struct as to make the code above (*) compliant to the Standard.

Otherwise the behavior is not consistent and hard to understand.

So, what's the reason for C Standard to consider const-ness recursively, as it puts it?