...if your friends like/comment them - anyone know why?

In the last few days, I've started seeing comments that my friends have posted on other people's walls appearing in my news feed - people I'm NOT friends with. I'm finding this a little bizarre (and rather creepy). Yes, the recipient's walls are public but this is still really bizarre.

Does anyone have any insight into what benefit there is meant to be in me knowing what my friends are saying to people I do not know? And, more importantly, has anyone worked out how to disable this in the Byzantine Facebook privacy pages? A quick search on the subject gets lost in the rest of the Facebook privacy discussions.