charfles.. it's because they don't know, you can't really figure out how much the person did after they od.
for most drugs, for most people, the amount you need to od is pretty high.
but there's always a chance something fatal could happen at a lower dose, especially with a drug like coke.
in all reality, given the huge number of people who use various controlled substances, those kinds of things are extremely rare.. i mean, huge numbers of drug users spend years doing incredibly stupid things that *should* technically kill them, but they don't die.
but some do, and you have to consider that.