Mike,
It sounds like a typical situation where the parents took your accusations against the daughter personally. In my experience, I have seen many times when the parents are more concerned about their reputation as the parent of the dirt bag, then the well being of their kid. I am not defending these people, but most likely they didn't want to be known around the community has the failed parents of a druggie daughter and this is why they wanted to know if you really had any solid evidence.
Now at this point you left them the opportunity to save face and deny it when their friends ask them about it. Also you should be prepared for them to trash you in their circle, making you out to be either crazy or a bad landlord. You got lucky and she left without a fight. Hopefully you will get just as lucky and the parents will go away too. I wouldn't want to get into a pissing match with these people, simply because it sounds like they are willing to defend their reputation and as you know, your evidence; now; is little to none.
I have had to actually throw the drugs that I found on their kids on the table in front of the parents to shock them into reality.