* "Foul" can refer to something that is dirty or unpleasant, like a deed.
* "Foul" can also refer to a rule violation in sports, especially in American football, where a "foul" might be called if a player trips another player.
The pun works because stealing ducks is a bad, or "foul", deed, but also because it could be interpreted as a "foul" in the sense of a sports violation, as ducks are often found near bodies of water where football games might be played.