My mother used to do that to me when I was a teen and it totally fucked me up, I don't know what she thought I was doing, but I don't think anything justify a person going over their kids diary or belonging. In worrying about my wellbeing and safety, she took both away from me. To this day I find myself lying for no good reason about things just to feel like I'm protected and carry about 20 pounds with of personal stuff in my bag everywhere I go.
I understand the need to protect your child from others, but know that by doing that you are actually taking away a lot of their sense of safety and actually, in a passive aggressive way - attacking them.
I think that unless there is a definite proof your child is doing something extremely dangerous,it's important to let them have the benefit of the doubt. We expect the legal system to give protection to inmates and criminals in ways we don't expect parent to respect their children.
If you are doing it, think a million times before why you are snooping and whether you can get the information in any other way. Just like you don't want your kid snooping on you, you shouldn't snoop on your kid.