Why does a function declaration override non-writable properties of the global object?

Setting a property descriptor like this:

Object.defineProperty(window, 'someFunction', {

    value: function() { alert('safe'); },

    writable: false,

    enumerable: false,

    configurable: false

});

...should, as far as I know, make the someFunction property of window non-writable. It works for function expressions as I expect, whether the function is directly assigned to the object property... ^fiddle

window.someFunction = function() { alert('boom!'); }

someFunction(); // safe

...or assigned to a global variable: ^fiddle

var someFunction = function() { alert('boom!'); }

someFunction(); // safe

However, it doesn't seem to work with function declarations : ^fiddle

function someFunction() { alert('boom!'); }

someFunction(); // boom!

Is this behavior intentional? What is the reasoning behind it? Is this documented anywhere? Or am I just making some kind of silly mistake?

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By the way, I'm using Chromium 17 to test this. Strict mode doesn't seem to make any difference.

Source: Tips4all, CCNA FINAL EXAM