Why does a const object requires a user-provided default constructor?

Question

Here is a piece of code:

class A {
public:
    void f() {}

};

int main()
{
    A a;       // OK
    const A b; // ERROR

    a.f();
    return 0;
}

This gives the following error: Default initialisation of an object of const type 'const A' requires a user-provided default constructor

What is the logic behind this ?