Why does hello world not compile in safe?

[Freddy via Digitalmars-d-learn]

----
import std.stdio;
@safe:
void main()
{
        writeln("Edit source/app.d to start your project.");
}

----
source/app.d(5): Error: safe function 'D main' cannot call system
function 'std.stdio.writeln!(string).writeln'