It was a medical marijuana case. California law says that people with a doctor's recommendation can grow and smoke marijuana. Scalia said (and yes, that was a quote) that Congress has banned the interstate market for marijuana, and that the only way to successfully do that is to ban possession of all marijuana everywhere by everyone, because otherwise marijuana legally grown in California could be shipped out of the state before anyone could stop it.
Seems like that ruling should make sanctuary cities absolutely illegal, because there is a chance that illegals could leave the sanctuary city before anybody could catch them and stop them...
Not to mention how it would allow the feds to be involved in unconstitutional electoral votes, which could also be shipped out of the state and impact the entire country.
Or does the principle of pre-emptive federal action only apply to commerce?