"What I'm saying is gthat plenty of people check themselves into rehab for drugs such as coke and heroin. What is it about pot that makes it different?"
Nothing about pot makes it different. Coke and heroin users who check themselves in to rehab almost always do it under duress, just like you said about pot users. They tend to be adults, so they don't need their parents to do it for them, but they don't have any better sense than pot users until the heat is on, and their attorney or employer lays down the ultimatum.
In fact, they pretty much all started out as pot users, who believed they could do whatever they wanted and somehow not pay the piper.
The rates of voluntary rehab are quite different. If you cannot concede this obvious fact, there is no use in discussing this.