Why isn't the [] operator const for STL maps?

Contrived example, for the sake of the question:

void MyClass::MyFunction( int x ) const
{
  std::cout << m_map[x] << std::endl
}

This won't compile, since the [] operator is non-const.

This is unfortunate, since the [] syntax looks very clean. Instead, I have to do something like this:

void MyClass::MyFunction( int x ) const
{
  MyMap iter = m_map.find(x);
  std::cout << iter->second << std::endl
}

This has always bugged me. Why is the [] operator non-const?