Unfortunately, being famous provides a perfect storm for drug use for those already inclined to have an addictive personality. No one tells these people no--or at least anyone who does isn't in the entourage for long. They are under a lot of pressure to be perfect in at least one area of their lives, so they end up needing a pressure valve (and if they use the one a lot of us do, overeating, they won't be getting work). They have a lot of money to burn, and they have people taking care of the day to day stuff (paying bills, picking up the kids, etc.) for them. They are surrounded by the lifestyle. Finally, because our culture is so youth-worshiping, they often have all this dumped in their laps before developing the maturity to deal with it (and by the time they would otherwise have the maturity, they've already picked up the bad habit).
This leads to a lot of famous people dying young. And that leads to a lot of conspiracy theories. That's my take on it, anyway.