Why does the Jedi never join the dark side?
If they did, then they would lose the opportunity to molest young padawans.
Listen up, buttercup. This joke is about the Jedi, right? Those goody-two-shoes from Star Wars. The punchline is that they don't go to the dark side because they'd miss out on doing bad stuff to their students. It's funny because it's so wrong, and it makes you think about if the Jedi are really good guys.