My parents found out I was doing drugs when one day I threatened to kill myself because they wouldn't let me go to San Diego. They called the cops (I was just being a dramatic asshole at the time, little did I know that if you threaten to kill yourself you have a mandatory 72 hour stay in a mental hospital) and while I was there I told the psychiatrist how many different drugs I had done, he was shocked. He convinced me to talk to my parents about it so I listed off the various things I've done, combinations of drugs, times I'd gone places with friends and had all day drug binges.
Anyway, the point of all this is.. after all that I stopped doing drugs except for weed, and they just kind of shut the fuck up cause I stopped coming home and terrorizing them at 5 in the morning.
They only told me that if they ever saw any pot or pipes or anything they'd kick me out. Except they always found random stuff, ash in my room, "What is that skunk smell?", even a pipe on occasion. They'd just throw them away if they found them.
Also, I tried the whole approaching them about it, talking to them about how responsible I am and that it's not as harmful as other things, blah blah. They said they don't really care because they're responsible for me and it's a risk to my safety. Most parents are never gonna let you doing something illegal.
So, basically it amounted to them not wanting to condone my use (like everyone is saying, not gonna happen) but being more accepting that I was just going to do it. That is about as lucky as your going to get, and it's not hard to obtain.