Rationale behind using namespace behavior

Quote from the standard:

A using-directive specifies that the names in the nominated namespace can be used in the scope in which the using-directive appears after the using-directive. During unqualified name lookup (3.4.1), the names appear as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace.

Look at this code:

namespace A {

    int fn() { return 1; }

}

namespace Inner {

    int fn() { return 2; }

    namespace B {

        using namespace A;

        int z = fn();

    }

}

Here, before I knew the exact rules of namespaces, I had expected that z will be initialized to 1, as I written using namespace A, so expected that A::fn() will be used. But it is not the case, z will be initialized to 2, as Inner::fn() is called because of the rule I quoted.

What is the rationale behind this behavior: "as if they were declared in the nearest enclosing namespace which contains both the using-directive and the nominated namespace"?

What would be the cons, if using namespace worked as applying using declarations for everything in that namespace?

Note: this is the related issue that motivated me to ask this question.