Jaak: Yes, it is a draw if both player have one piece left, even if one piece can capture the other.  The reason is that the object of the game is to get two pieces into the opponent's castle.  The reason that the capturing of all of the opponent's pieces while having two or more of your own pieces left is defined as a win is simply because those two pieces can proceed unhindered into the oponent's castle.  This definition avoids the silly process of moving pieces move after move while your opponent just sits there.

However, one piece vs. zero pieces is a draw, not a win, because it is not possible to castle two pieces if you only have one.