Interesting...it has never once occurred to me to ask permission from someone before posting a pic of them (well, I mean, it's never just them, but them and my kid, or them and me, etc.). I do tag them, however, so that they can see that I posted the pic, and they can un-tag it if they wish.
However, if anyone ever asked me to take down a picture for any reason, I would, no question. And I absolutely don't think that asking me to take pics down deserves defriending.
But I keep my privacy settings locked up pretty tight and my friends list pretty small. If my page was public, of course I would ask before posting. But seeing as the only people who will see it are people who, as a PP said, I would show a photo album to if they were in my house...well, I just don't worry about it that much.