To play devils advocate, there are probably police officers and health officials who need to know this info to recognize users and those about to use. Plus a lot of the book is about how to deal with OD's and HIV users. Really, how many 22 year old fresh-from-college healthcare workers are going to know the diff between a person shooting H and injecting insulin for diabetes. If you're going to have the power to take away ppl's kids, etc, you need to know what drug use looks like.