I think that is because or society just does not approach drugs in an honest or healthy manner. Caffeine is very widely abused and addiction runs rampant throughout society, and it's not even thought of as a drug. Alcohol abuse is tolerated, accepted, and even encouraged on certain occasions, it's rather addictive, but yet we put it in its own category when say "drugs and alcohol." We also think of nicotine addiction as its own separate category, and call them "smokers" who may or may not "quit," and seldom do we accept and embrace they too have a drug addiction. A few OTC drugs in large enough doses can cause recreational highs. But you'll never be shamed for taking Benedryl for your allergies or cough syrup when you have a nasty cold. They can get you really high though - I've heard Benedryl can even really make your mind flip with very vivid hallucinations, and descriptions often revolve around the idea that one is living in a horror film. I've never used it for such an effect, but I'll sure take a couple pills when my allergies are flared up.
"Evolution at work" made us social animals and care for people, and even us who are empathy-impaired still care about others and are concerned with how people are doing. And in many cultures, drug addicts (real addicts/junkies, not the average occasional user) do not receive this "you piece of ****" attitude, but rather they are cared for, helped, and tend to find themselves in much better position for recovery. Whether you like it or not, we are social animals--apes to be specific--and we survive as a group, not as individuals. Mutual cooperation evolved us, not everyone for themselves.
Apparently loosely paraphrasing from research documents and publications makes you an addict according to some.
When I went to Colorado, it made me a stronger supporter of medical use, because when it's fresh and hasn't been vacuum sealed and not handled with care, you don't need to use as much, and you don't need to get stoned to get the medical benefits. Do some people just want to get stoned? Sure. Does everyone who drinks wine or beer abuse them and get drunk? No.
Why this hyper obsession with dismissing sweeping all drug users into this huge lump of addicts? Do you think when these topics are formally studied researchers aren't watching out for signs of addiction? It may be hard to accept this, but even advocates such as myself sometimes look at people who bring up driving under the influence like they've just asked something very bizarre, because it's kind of a "no duh" thing like driving drunk.