What is the funniest joke here?

Ever heard the story of George Washington and the cherry tree? He chopped down his father's favorite cherry tree. His father asked him if he'd done it, and he said yes, and his father forgave him because he told the truth. Okay, now let's get to the joke:

A kid was walking by a river when he saw a porta-potty standing near the edge. He had a sudden urge to push the porta-potty into the river. He made no effort to resist this urge, and so the porta-potty ended up in the river.

That night at dinner the kid's father angrily storming into the dining room and took his seat without a Word. After a while he said, "Hey, son, ever heard the story of George Washington and the cherry tree?"

The kid burst into tears. "Yes, yes I know it, and yes, I'm the one who pushed the porta-potty into the river!" he cried. "Now will you forgive me because I told the truth?"

"NO," his father growled angrily, "because George Washington's father was not in the tree when he chopped it down!"