Because, unlike us, they kids are raised in an alcohol friendly environment. Whilst they are allowed our teens have probably have chances far and between, and seeing that these times are few, they binge. It makes sense really, when you prohibit someone from doing something, when they get the chance to actually do it, they abuse it.
Stripping privacy away from your kids as they get older, as opposed to trusting them more, while having the obvious side effect of them being really nit-picky about their privacy when they grow up, will cause them to abuse the things that they have few chances to actually do. Now of course I seriously doubt it'll go to such an extreme, but it IS something to think about.