State legislatures repealed the laws against it that they had. After that it’s no distinction at all. State law enforcement was directed to not enforce federal law, because they’re not required to.
There’s no big constitutional issue: states cannot be compelled to enforce federal law, this is already clearly settled. This is why things like the age to drink alcohol are forced onto states by withholding highway funding, the federal government can’t pass a national 21 and over law and expect it to be enforced, so they told the states “pass your own over 21 law or we won’t give you federal highway money” which is constitutional they say under the equal protection clause because the condition applies to all states, and the courts have ruled this way. Now every state has their own individual 21 and over law, something many states didn’t want to do. This has to happen because state law enforcement cannot be compelled to enforce federal law.
These sheriffs, and the state gun law repeals and the silencer laws and all that kind of stuff are applying the same concept. Legalizing weed is really what kicked this movement into high gear, it was the first time in a century or more that states managed to prominently just not enforce federal law, now it’s become a broader movement. Nobody but feds are required to enforce any federal law, they’re just required to abide by the constitution.