walk the plank

Reader's Digest 11月号に掲載されていた次のジョークの意味が分かりません。
As a child, I was made to walk the plank.
We couldn't afford a dog.
"walk the plank" の意味が分からないので当然ながら "We couldn't afford a dog." が何故オチになるのか悩みます。　"plank" の二つの意味（1. a long, flat piece of timber, thicker than a board. 2. any one of the stated principles or objectives comprising the political platform of a party campaigning for election）は知っていますが、ここではそのどちらの意味でも無い様なので、"walk the plank" が慣用句ではないかと思い調べると、確かに面白い意味がありました。
・Dictionary.com:
a. to be forced, as by pirates, to walk to one's death by stepping off a plank extending from the ship's side over the water.
b. to relinquish something, as a position, office, etc., under compulsion: We suspect that the new vice-president walked the plank because of a personality clash.
・Merriam-Webster's Online Dictionary:
1: to walk under compulsion over the side of a ship into the sea
2: to resign an office or position under compulsion
・McGraw-Hill Dictionary of American Idioms and Phrasal Verbs:
Fig. to suffer punishment at the hand of someone. (Fig. on the image of pirates making their blindfolded captives die by walking off the end of a plank jutting out over the open sea.) Fred may think he can make the members of my department walk the plank, but we will fight back. Tom thought he could make John walk the plank, but John fought back.
引用したジョークはこの慣用句の意味を比喩的に使用していると推測しますが、それでもオチが "We couldn't afford a dog." ではどこかの最貧国の親に捨てられる子供のようで、ジョークにはなりません。
ブラックジョークだとしても面白くはありません。　何か別の解釈があるのでしょうか？
但し、慣用句の "walk the plank" は覚えておこうと思います。