Why do people love to say that correlation does not imply causation?

Daniel Engber in Slate:

Depressed people send more email. They spend more time on Gchat. Researchers at the Missouri University of Science and Technology recently assessed some college students for signs of melancholia then tracked their behavior online. “We identified several features of Internet usage that correlated with depression,” they said. Sad people use IM and file-share. They play video games. They surf the Web in their own, sad way.

Not everyone found the news believable. “Facepalm. Correlation doesn't imply causation,” wrote one unhappy Internet user. “That's pretty much how I read this too… correlation is NOT causation,” agreed a Huffington Post superuser, seemingly distraught. “I was surprised not to find a discussion of correlation vs. causation,” cried someone at Hacker News. “Correlation does not mean causation,” a reader moaned at Slashdot. “There are so many variables here that it isn't funny.”

And thus a deeper correlation was revealed, a link more telling than any that the Missouri team had shown. I mean the affinity between the online commenter and his favorite phrase—the statistical cliché that closes threads and ends debates, the freshman platitude turned final shutdown. “Repeat after me,” a poster types into his window, and then he sighs, and then he types out his sigh, s-i-g-h, into the comment for good measure. Does he have to write it on the blackboard?Correlation does not imply causation. Your hype is busted. Your study debunked. End of conversation. Thank you and good night.

More here.