I'm a little confused about why a parent wouldn't let their teenage daughter lock the bathroom door!? I also don't think it's terribly realistic to worry about BB guns and snack packs being laced with poison. I'd love to know how many times you have found that your kids squirt gun is actually a real gun, or discovered poison in your kids snacks or BB's.
Even my parents don't lock the bathroom doors when they go in. For our household it has to do with safety and not having to break down the door if someone had to get in (think medical emergency). We have had a little bit of a problem with my 3-year-old coming and going whenever he wants but we have gotten that to the point where now the only person he walks in on is me. Still working on that one for some reason Mommy apparently doesn't need any privacy.
As for the OP: As my son's room is my room currently he really wouldn't have much say about me looking through it if he was old enough to be concerned. In the future I think that I will continue to peek in on him when I go to bed, going in to get his laundry is a given LOL he is a boy after all and I doubt that laundry will get into a basket (I can try but who knows). He will have to clean his own room etc. That said if I thought that he was drinking or doing drugs it is my house and I will look for proof and talk to him about my concerns. I hope that I will raise him better than that and that I will never have to go searching but you never know and I am not one to say that I would never do something. LOL
In my house is for the same reason you never know what could happen that may require you to have to go in, but then my girls have no problem with that, they have taken the fact since they were babies, that the doors are not locked. We do not walk into anybody private business, we see the door close they light on, somebody is in there and it is left alone, then again everybody walks in on me to ask for money, where is this item or that item, thank god for thick curtains LOL.
As for the rest of the house, teenagers like to do things that they are not allowed, it is the nature of the beast, so my kids and their friends know if you come to my house, as long as the doors are closed but not lock you never know when I might walk by and decide to check on you!The one and only time a door got locked in my house my daughter was 8 with 2 friends and I discovered that they had accidentally gone into a *bleep* site when they typed the wrong search and they locked the door so they could see it, and all hell rose later. And then 1 time when my teenage daughter throw a party for 13/14 years old and they decide to turn off the lights and locked the doors, needless to say that everybody went home 10 minutes later.