It's also why I have the "I will monitor your internet usage at any time that I feel it's necessary" discussion. I don't like invading privacy - I wouldn't want my internet history explored and it's as vanilla as you can imagine but it's also a projection of my inner thoughts.
I knew a young girl who was lured into a sexual encounter with an adult after striking up a friendship on "Words with Friends". I don't trust/allow anything with private communication.