If marijuana is lawful to carry in both the city of origin and the destination, the TSA should allow passengers to carry it. However, since this issue is still relatively new and still being addressed in many places, it places an undue burden on TSA agents to know which cities have ruled it legal. Thus, the TSA should simply allow passengers to carry it in small amounts and let the passengers be responsible for the legality of their actions.
The TSA should absolutely let you take marijuana on planes, as long as you are traveling between places where it is legal. For many people, marijuana functions as an irreplaceable medicine, and it would not be right to make a person do without their medication if state law says it is legal.
Not all states have legalized marijuana. Why are they, anyway? It makes little sense to me that there are increasing restrictions on cigarettes but not on marijuana. One place where cigarettes are restricted? Airports. And planes, too. If cigarettes can't be taken into an airport, then why should marijuana be allowed?