I think that teenagers need a little bit of privacy and a little bit of freedom in their lives. If a parent is always watching what their child is doing then the child is going to grow up to be a sheltered person who doesn't understand real world problems. But then if a kid has no supervision and can do what ever they want, then it is easy to live your life down the wrong way. Without parents telling you what to do then you could make bad decisions with drugs and things of that nature.
If a parent trusts the kid enough to let them do somethings, but then thinks the child needs to be watched sometime then thats a good thing. But the child needs a little bit of trust in him so he can start to trust him self