Because while the rest of us can just shake our heads and close the window, there is a certain sort of person that LOVES pointing out when people on the internet are wrong. They get a real charge out of it every time they do it, so they keep doing it.
No, seriously, this is the answer. There are pleasurable brain chemicals released and everything.
But simple possession is a local charge. How do the feds get involved? They find a bag of weed while searching through your stuff at the airport, is all I can figure.
In Denver airport they don't even search for it, so many people leave with legal weed and weed products.