That some of them are pictures of women in people's regular lives that are totally unaware that there picture is up?
For instance, I don't like Suzy Lee. I get this phone number to an offline fax machine and take pictures from her facebook page and post it on backpage with that number just for kicks.
Now, I wouldn't actually do that but with so many pics flying around wouldn't you think that some personal pictures have landed on BP every once and a while?