could it be that they were banned from google adsense at one time and now they're mad at the world. jealousy? revenge...? just chaos maybe? effing with someone they don't know for no reason other than to eff with someone. I'm sure all of the things I mentioned are common reasons. Do you have any tales to tell?
DP is a good example... which is the primary reason I do not include my site in my signature like most do. I also do not want crappy traffic from DP anyway.