The reasoning here is the same as photo tagging, or wall posts. Friends can tag "you" in photos without your explicit acceptance of the tag. You have a much mor

The reasoning here is the same as photo tagging, or wall posts. Friends can tag "you" in photos without your explicit acceptance of the tag. You have a much more powerful tool for preventing "prank" photo tags/wall posts/group joins than Facebook could ever invent technologically: social pressure. To wit, these people are nominally your friends; even if you've been somewhat promiscuous, and they're just your coworker, or acquaintance, or distant family member, you still have much more leverage over their behavior than Facebook ever will. So, if Joe Idiot tags you in a photo of a goat, or joins you in an embarassing group: a) untag/unjoin yourself, and b) tell Joe THAT WAS NOT COOL. Optionally, unfriend Joe. I.e., exactly what you would have done if Joe wrote a blog post about your embarrassing medical problem, or put you on a mailing list you do not like.