Crossing state lines with marijuana elevates your offense to the Federal level, regardless of the legality situation in the source and destination states.
Federally, marijuana transport across state lines is still trafficking in illegal narcotics. That the States don't help enforce it doesn't change a thing.
Not a lawyer, just read books, mind.
* If you buy something legal in one state
* and you then travel to another state (where its illegal) with that item
* then you have broken the law and can go to jail.
Its not the doing something in state-a and then traveling to state-b. Its the traveling to state-b with something illegal in state-b.
Does that make more sense?