Great question. First off, If you want to avoid guys jacking off, Sextfriend posted a list of all the sites like Omegle and Dirty Roulette you can use to chat with other people and not worry about unwanted sex acts. The answer is that a lot of people have something called exhibitionism, which is a mental disorder characterized by a compulsion to display one's genitals to an unsuspecting stranger. These people often turn to online chat and video websites because it fulfills that sexual need of being watched naked by other people. From what I understand, guys who go on those sites to masturbate get thrown off by the admins after a while. I hope this answered your question.