It is a basic, intuitive, premise of our federal system that each state makes law for that states and not the other states. To steal an example from Professor Don Regan, even if the state of Michigan had a very strong anti-smoking policy, it could not ban the manufacture of cigarettes in North Carolina, only the importation of those cigarettes into Michigan.

And yet, it is not clear what actual part of our positive law enacts that premise. There is no Territoriality Clause in the Constitution, no Territoriality Statute, and arguments based on other more esoteric sources of law (general law?) raise deep questions of their own. Moreover, as every conflict of laws scholar knows, the basic premise of our federal system is frequently denied in practice—states regularly apply their own law to conduct that took place in other states, with relatively little shame or theoretical explanation. And if there is no Territoriality Clause in the Constitution, maybe they are allowed to do that.

On occasion, the courts get so flummoxed with this that they temporarily press other constitutional clauses into temporary active serve as a Territoriality Clause. The Due Process Clause is occasionally asked to do this work, for instance (as it as asked to many constitutional odd jobs). More promisingly, several cases had asked the so-called dormant Commerce Clause to do this work as well. If states cannot discriminate against interstate commerce, the logic went, then they also cannot simply regulate out-of-state commerce either.

Read more at The Volokh Conspiracy