Assignment and const members

Consider the following code:

#include <iostream>

class MyClass
{
    const int value;

public:
    explicit MyClass() :
    value(10)
    {
    }

    MyClass(int val) :
        value(val)
    {
    }

    int GetValue()
    {
        return value;
    }
};

int main()
{
    MyClass a, b(20);

    std::cout << "a.value = "<< a.GetValue() << " b.value = "<< b.GetValue() << std::endl;
    b = a;
    std::cout << "a.value = "<< a.GetValue() << " b.value = "<< b.GetValue() << std::endl;

    return 0;
}

When built with Composer 2013 SP1 Update 4 on Windows, I get:

a.value = 10 b.value = 20

a.value = 10 b.value = 10

Two questions:

1. Why does this compile? Neither g++ nor MSVC will compile it because of the const member, which cannot be overwritten in the default assignment operator.

2. Why does assignment modify the const value?