> California, Colorado, New York and Oregon off the top of my head
All decriminalization is not alike. In New York, for example, decriminalization is in in name only, because police can easily exploit a loophole that allows them to charge people with possession of more than an ounce (criminal) even if they only have a few grams (not criminal). Because of this, New York City arrests more people (per-capita and in total) than any other city in the entire world. Compare to the Netherlands, which also has 'only' decriminalized marijuana.
In New York, the police will do a stop and frisk, tell people to empty their pockets, and then charge people with displaying the drug in public when they obey the order to empty their pockets. Is this the loophole you mean? Or is your loophole a different one?
More or less. There are other variations on how to exploit the same clause in the law, but basically, marijuana that is 'open to public view or burning' can be prosecuted as if it is 28+ grams, regardless of the actual amount.
The history behind how New York's marijuana decriminalization and this loophole came to pass, especially in contrast to the Rockefeller drug laws, is actually a rather fascinating example of the sheer racial/socioeconomic hypocrisy within US drug laws.
If it's for medical use and you're all registered as a patient, it's now technically legal under local and state laws, but illegal under federal law.
TL;DR the laws are a mess and if the FBI catches you, you're still pretty screwed. We need to work on reforming the laws at a federal level, since that overrides all lower laws. It'd be at least cool to have the federal law just refuse to say one way or the other on marijuana, so that states could decide what is right for them.
Does that indeed override the state laws? Many states have laws against things that are not federally prohibited, and legal in other states that do not prohibit those things.
States can be more restrictive but they cannot be less restrictive. Thus if the nation declares that marijuana is illegal a state cannot declare that it is legal (see the medical marijuana raids in California). And if the nation declares that alcohol is legal then a state can make it illegal within that state.
And this is the opposite of what the founders would have wanted, I'd guess. The constitution mandates that powers not specifically granted to the federal government are reserved by the states. I'm no constitutional scholar, but I'm pretty sure the federal authority to regulate drugs was "found" in the interstate commerce clause as part of the food and drug reforms in the early 1900s. At the time, there were definitely problems but like all big government intervention the cure itself is now out of control and we now have an agency (FDA) that has its hands in about 1/4 of the GDP.
This is why the first two federal laws targeted at prohibiting drugs didn't actually do it directly - they did it by way of taxes. The Marihuana Stamp Tax Act (1937) is the most famous, but it was preceded by another act that targeted proper narcotics (morphine, etc.). I believe it was the Harisson act, but I'm not certain.
In any case, the 1937 bill created an impossible-to-satisfy tax structure, because they knew that a direct prohibition would have been deemed unconstitutional. Unfortunately, by 1970, everyone had forgotten and the Controlled Substances Act passed easily.
This is sorta-kinda true, but not quite. While the residents of a state are technically held to the union of state and federal laws, the jurisdictional issues are important. If a state declares something legal, that means that state-level LEOs won't enforce the federal laws. Those laws are still in effect, but if you're a deadbeat twenty-something toking up behind the liquor store, you care about the subset of laws being enforced by the local beat cop, not those being enforced by the FBI.
All decriminalization is not alike. In New York, for example, decriminalization is in in name only, because police can easily exploit a loophole that allows them to charge people with possession of more than an ounce (criminal) even if they only have a few grams (not criminal). Because of this, New York City arrests more people (per-capita and in total) than any other city in the entire world. Compare to the Netherlands, which also has 'only' decriminalized marijuana.