First you're wrong about the message that the parents get. As it stands the message that they get is, We're not at liberty to discuss this topic. They neither get confirmation or denial that sex and drugs came up as issues. They get a big, blank slate. They don't get told, There is a problem, but we won't tell you what. They get told nothing.
Review the case in point. The child gave out the clue that there was a potential problem. Nothing that the clinic said gave that away.
Now your policy is basically an open invitation for suspicious parents to go on "fishing expeditions" to see if something is up. Which completely defeats the point of having confidentiality here in the first place.
Again, consider the case where the parents run a drug operation, or the child is being sexually abused by a parent. Do you really think that the parent should be told what the child said? And before you suggest that the policy be to restrict what is said in that case, be aware that this is a signal to the parent that their kid said something they shouldn't, and further the clinic may not know of the issue, but telling a seemingly innocuous detail to the parent might trigger rage.
Cheers,
Ben