This rehab stuff reportedly all stems from the rookie symposium deal last season where he got caught in a room with a couple of other rooks and some weed. No one is sure whether or not he got caught again after that, or whether just the symposium deal was enough to get him to agree to rehab.
Either way this is supposedly all about marijuana, which is far less addictive than "hard drugs". There are people in the league right now who have admitted to smoking it in the offseason, and many more who haven't admitted it and get away with it.
Beasley has made some poor and immature decisions, but he is a 19-20 year old guy that suddenly got a ton of cash and a high profile job. Not all players know how to deal with that, some take longer than others. Up until the rehab story broke, no one had anything but praise for him working extremely hard this offseason.
Also, the Heat aren't going to sell Beasley for any fire sale price whatsoever because of this. Not yet anyway. You generally don't trade a player at their lowest value without giving them a chance in actual games to redeem that value. They will see what happens this season first, then re-evaluate after they see who they are able to sign next offseason with their cap room.