Well, since "federal law can't impose on state law" has been the SCOTUS party line for the last X years, we might as well use that line of reasoning here...
The exception proves the rule, however.
My understanding is that cannabis regulation in the United States is mostly predicated on a case where a farmer grew his own non-cannabis, I believe it was wheat, and then the government compelled everyone to sell their wheat at a specific price, which he did not want to do. He protested and I believe turned the wheat into other products, which caused this court case.
The result of the case was that the government was able to justify use of the Interstate Commerce clause because the actions of NOT selling the wheat could cause economic disruption.
The case is Wickard v. Filburn (1942)
This was essentially re-upped in 2005 with Gonzales v. Raich (2005)
Wickard v. Filburn established "activities local in nature can still be regulated by Congress if, in the aggregate, they could substantially affect interstate commerce."
The effects of this ruling are broad. Like all things there are plenty of things that as it stands right now are built on top of that legal "system" like bills that enforce clean drinking water, but those can be done a different and correct way and we don't need the government using this method to accomplish that.
It's possible they are avoiding the marijuana fight in order to keep this broad interpretation, though. I have a pet theory that the equal rights amendment isn't in the constitution to avoid a legal fight that would also strike down the income tax, but these are all sort of just theories.
> The powers not delegated to the United States by the Constitution, nor prohibited by it to the States, are reserved to the States respectively, or to the people.