Why returning NULL from main()?

I sometimes see coders that use NULL as return value of main() in C and C++ programs, for example something like that:

#include <stdio.h>

int main()
{
    printf("HelloWorld!");

    return NULL;
} 

When I compile this `code with gcc I get the warning of:

warning: return makes integer from pointer without a cast [-Wint-conversion]

which is reasonable because the macro NULL shall be expanded to (void*) 0 and the return value of main shall be of type int.

When I make a short C++ program of:

#include <iostream>
using namespace std;

int main()
{
    cout << "HelloWorld!";

    return NULL;
}

And compile it with g++, I do get an equivalent warning:

warning: converting to non-pointer type ‘int’ from NULL [-Wconversion-null]

But why do they use NULL as return value of main() when it throws a warning? Is it just bad coding style?

• What is the reason to use NULL instead of 0 as return value of main() despite the warning?
• Is it implementation-defined if this is appropriate or not and if so why shall any implementation would want to get a pointer value back?