And penicillin can kill, if you have an allergy to it, which is one reason why it is a prescription only (and therefore controlled) drug, used under medical supervision.
It is also a therapeutic drug, but
AFAIK, mephedrone has no therapeutic use - unless you happen to be a plant.
Other class B drugs and class A drugs also have therapeutic uses, morphine for example, and cocaine, which is why they can be used legally - provided they are prescribed by, and used under the supervision of, a registered medical practitioner.