It bothers me so much. And now there's one going around that says "Like and Share if you would help Jesus carry the cross. Ignore if you love The Devil." It's just really annoying. And I guess the point is to show people how compassionate you are or something.
The same reason they send them via email - chain letters are emotional manipulators, and people who fall for them don't even realize or care they are being played and roped into some scheme to spread particularly annoying spam started by some anonymous hoaxer.