Smearing every substance that happens to be illegal in a certain country at a certain time with the pejorative label "drug" both unfair, ignorant, and counterproductive.
Good arguments could be made for considering alcohol, coffee, and cigarettes "drugs", and yet most people don't think of them in that way.
In fact, in some societies alcohol, coffee, and tobacco were outlawed -- even to the point of instituting the death penalty for their posession or use.[1][4] In some societies, the posession or consumption of alcohol is still forbidden; while in others it's quite commonplace and not considered a big deal.
Coffee drinking has been accepted nearly everywhere as a virtually harmless or even beneficial activity. But look at the fear and outrage it generated in earlier European society as evidenced in Bach's humorous Coffee Cantata,[2] or in the Middle East where for a time it was made illegal.[4]
Perhaps one could argue that opium is worse than coffee, alcohol, or tobacco, and so deserves to be called a "drug" and made illegal. But counter arguments have been made that at least alcohol and tobacco are actually worse, and should be made illegal if any of these substances are. Alcohol actually has been made illegal in the US in the past and that hasn't worked out so well.[3]
And where does all this leave psychedelics, many of which have been shown to be far less harmful than opium, alcohol, and tobacco? In fact, many studies have shown the benefit of psychedelics when used constructively for therapeutic purposes.[5] There's also a long history of psychedelic and other currently illegal substance use in religious ritual, and of the belief that these substances are sacred.
Should people go to jail for helping themselves and others, for peacefully practicing their religion, or for seeking to expand their minds and awareness through these substances? Should these substances be called "drugs", "sacraments", or "tools"?
Advocates of the so-called "war on drugs" often will not accept or even acknowledge the possibility of constructive, positive currently illegal substance use. They tend to smear all such use as drug abuse.
Yet much of what society values: music, art, literature, religion, technology, science, and medicine has been positively influenced by currently illegal substance use.
To take one relatively uncontroversial example: odds are that much of the music that most readers of this post listen to every day was inspired directly or indirectly by these substances. It might not say that on the album cover, but dig in to it a bit and you'll see that either the musicians who made the music themselves used these substances to inspire their music, or the musicians who strongly influenced them did. The same goes for a lot of art and literature. This leads to another important point: much of the good that currently illegal substances do is unacknowledged because there's a stigma against admitting that you use or approve of them.
If you look at the history of mind-altering substance use and the reaction of socieites to these substances, a clear pattern emerges where it is usually new mind-altering substances that are feared and made illegal, while mind-altering substances that have had a long history of use in that society are tolerated or even considered sacred (as in the wine drunk during Catholic rituals, or coffee's use by Sufi mystics[4]).
Another feature of drug prohibition is that there are often religious, moralistic and puritanical aspects to reactions against certain substances. Some altered states of consciousness achievable through the use of these substances are considered to be "impure" or "derranged" in some way, and their users are considered immoral, depraved or wicked. Anti-drug crusaders often adopt a moralistic, "holier than thou" attitude towards users.
Racism and xenophobia are other common features of the drug war. Substances which minorities choose to use (like crack used in black ghettoes) are stigmatized, while substances which the dominant majority chooses to use (like cocaine use in white suburbs or on Wall St) are effectively tolerated.
Political opportunism plays a huge role, as does the eagerness of politicians to appear like they're being "tough on crime".
Finally, there's the economic aspect -- where the prison industry, police, lawyers, judges, and the Drug Enforcement Agency benefit from arrests, imprisonments, and asset forfeitures. The alcohol and tobacco industries, and arguably the pharmaceutical industry also stands to benefit from keeping certain substances illegal.
[1] - Just one example of many: "c. 1650 The use of tobacco is prohibited in Bavaria, Saxony, and Zurich, but the prohibitions are ineffective. Sultan Murad IV of the Ottoman Empire decrees the death penalty for smoking tobacco: "Wherever the Sultan went on his travels or on a military expedition his halting-places were always distinguished by a terrible increase in the number of executions. Even on the battlefield he was fond of surprising men in the act of smoking, when he would punish them by beheading, hanging, quartering, or crushing their hands and feet . . . . Nevertheless, in spite of all the horrors of this persecution . . . the passion for smoking still persisted.'" - http://www.trivia-library.com/a/history-of-legal-and-illegal...
[2] - https://en.wikipedia.org/wiki/Coffee_Cantata
[3] - https://en.wikipedia.org/wiki/Prohibition
[4] - http://www.bbc.co.uk/news/magazine-22190802
[5] - https://en.wikipedia.org/wiki/Psychedelic_therapy