I don't know that it's completely understood, even by the rehabs, as to just how dangerous it can be to remove a substance when a person is very deeply in the throes of addiction. If you consider that given enough time and exposure to the substance the organism tries to adapt to it at a cellular level.
Take the substance away and everything really goes haywire as the organism is now trying to function without the substance that it has adapted to. Overall function of the brain, heart rate and rhythm, etc.etc., just a ton of things that could go awry. DT's, seizures and the withdrawal syndrome are just an expression of the body trying to normalize. When you stop and consider all the organ systems that have to readapt to function without the offending substance present it isn't that hard to figure out that some won't make it thru the process.