In the U.S., Law enforcement favors going after the distribution of narcotics over the use. Going after the users just means dealers will seek out new customers whereas going after dealers means eliminating the supply chain. And Law Enforcement would much rather have a dealer flip on a manufacturer at that. The bigger the fish they bring in, the more damage they can do to the supply chain.
The U.S. has recently been moving towards treating the use of illicit substances as a disease rather than an crime, especially when the use is tied to addiction. To this end, the current trend is to get the users to clinics that can specialize in breaking addiction rather than jailing a user. Prisons and jails do not have a strong track record for this, but they do have programs for substance abuse.
The U.S. also favors plea bargins rather than taking every case to trial. 90% of all legal cases are settled out of court with a negotiation between the prosecution and the defense. Usually this acts in a "you scratch my back" arrangement where the offender will plea to a lesser charge and in exchange, help the investigators find that bigger fish. This may be testifying against them or giving them the supplier or even acting as a mole. This also benifits the state as the prosecution doesn't have to pay for all the costs of arguing a case a trial. For example, if the cops bust a user, he will be charged with Possession, but they might drop charges if the user can name the dealer and will testify against him. If the cops bust the dealer, the dealer is charged with Possession (with intent to sell), but if he's low on the totem pole, they can ask him to name the supplier he gets his goods and testify against the supplier and in exchange, they drop the charge to Possession. They will be less inclined to bring this fact up, but Possession is easier to prove in court than Possession (With Intent to Sell) because the former requires finding drugs on the person and his property. The later needs to prove that the drugs were not for personal use but instead for distribution to others. Almost universally, anyone convicted on mere possession charges probably did something a lot worse. Law enforcement tends to charge as many crimes as possible... as long as they can prove guilt on one, they can put someone away for a while. Al Capone famously went to one of the most notorious prisons in the United States for Tax Evasion (a very white collared crime that typically results in fines and very minimum jail time. While we're on the subject, the IRS has a 98% conviction rate, and ties with Secret Service (the guys who protect the President) for most successful Federal Law Enforcement Agency, to give you an idea of where the priorities lie in the United States. All Federal Law Enforcement is ridiculously good, to the point that bringing charges against anyone is almost as good as saying they're guilty).
Another reason Use of drugs isn't pushed is that, well, producing that evidence is difficult. The common user bust is drinking and driving (since alcohol is legal in the U.S., use based crimes are the top level of enforcement. You legally cannot hold a firearm in the U.S. while drunk, even if you could do so while sober). A brethalizer is built to measure the amount of alcohol is in one's system and if it is over the limit for what is safe. Most illicit drug tests don't do this and rather look for chemicals in general and will ping if there's any trace... even if the last use was a month ago. Certain substances linger for a while, especially if they can deposit in hair folicle. You could be six months sober and still ping the test. Other issuses include drugs that contain components of legal substances. Opiods (the big problem drug right now) are manufactured from the Poppy Plant... which also is grown for legal consumable products. There are cases where the Opiod tests pings positive on people who have consumed Poppy-Seed Bagels (perfectlly legal and quite common) for breakfast the day of the test. Other tests aren't reliable and will give a false positive from time to time.
Edit: Typically, Possession of a controlled substance assumes you will use it. Reasonable Doubt is harder to make for having the stuff then it is for using it. Someone under the influence could claim they were drugged and law enforcement would have to prove that they weren't to convict (There's actually a known problem where law enforcement agents can get dosed by accident while handling contraband evidence. There was an episode of CSI: Miami where one character had this happen to her... and they realized where the drugs were hidden in a warehouse she served a warrant on but didn't find any evidence. DVD extras featured a police consultant who explained that the whole sequence was thought up because it happened to her for real.). Having the stuff in your possession is much more difficult as they have to show you had it (someone slipping it into your pocket without your knowledge is your job to prove).
In the U.S., Prosecution Discretion means that the Prosecutor can choose not to press the case for any number of reasons, they don't have the resources to fight it, to they don't think they can win with the evidence they have, to political motives (they don't agree with the law... this can be risky depending on the nature of the crime). Typically a simple possession charge where the amount is clearly small enough that it's only for personal use and no other crimes are charged wouldn't be worth the amount of resources to fully prosecute. TO give an example, while Marijuana is legal in the State of Colorado, it's still illegal under Federal Law, so smoking for recreational use in Colorado is still a crime, but not one enforced by the State of Colorado but the Department of Justice (U.S. Federal Law enforcement department... usually). Remember the Feds generally deal with bigger crimes than someone getting high under the bleachers while listening to Jimmy Hendrix, so it only really comes up if you did something bigger (kidnapping a girl, driving over state lines into Colorado, go to a school to get high under the bleachers while listening to Hendrix would get you a Federal Possession charge... but the real concern is the Kidnapping, not controlled drug use). And most crimes that occur in entirely one state, Feds are content to let those states deal with the problem and rarely get involved.
