Most everyone will be able to understand the logic of this one. You can also use this handout to help people understand that not all use is abuse. Also, that the vast majority of drug users are not abusers or addicts, similar to alcohol drinkers.
If they say something like, "why does society need another drug for people to use or abuse?" Then please point out to them that logic rests on a false assumption, namely that these drugs are not already available.