What if the lambda expression of C++11 supports default arguments?

I think the following code is very handy and no harmful:

auto fn = [](bool b = false) -> int // NOT legal in C++11
{
    return b ? 1 : 0;
}; 

Why does C++11 explicitly prohibit default arguments of the lambda expression?

I just wonder the rationales and considerations behind.

I want to know "WHY" rather than "WHAT" the C++11 standard says.