Why I must put the "operator<<" reload function into namespace std to

Why I must put the "operator<<" reload function into namespace std to
use ostream_iterator .

My compiler is VC2005.

Code As Below

#include <map>
#include <string>
#include <iterator>
#include <iostream>

using namespace std;
namespace std
{
       //If don't put this function into namespace std , there will be
a
compile error .

       template<class _Elem,class _Traits,class K,class V>
       basic_ostream<_Elem,_Traits>& operator<<(
               basic_ostream<_Elem,_Traits>& s
               ,
               const pair<K,V>& p
               )
       {
               return s<<p.first<<" : "<<p.second;
       }

}

template<class _Elem,class _Traits,class K,class V>
basic_ostream<_Elem,_Traits>& operator<<(
       basic_ostream<_Elem,_Traits>& s
       ,
       const map<K,V>& m
)
{
       copy(m.begin(), m.end(), ostream_iterator<pair<K,V> >(s,
"\n"));
       return s;
}

int main()
{
       using namespace std;
       map<string,string> a;
       a["key1"]="value1";
       a["key2"]="value2";
       cout<<a;
       system("pause");
       return 0;
}

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