It doesn't matter if the weed was purchased legally somewhere else, if police have a reasonable belief that someone is carrying illegally in their state, they should be able to stop them. People can legally purchase fireworks in states, but they cannot bring them to a state in which they are illegal.
There is a reason why some states are not making marijuana legal. People buying and bringing marijuana from other states where it is legal are basically breaking the law in the state they are bringing it. So police definitely have the right to investigate if someone is suspected of breaking a known law. There is nothing wrong in following the rules.
Marihuana is something that should be legal all over the country. The fact that it is legal in certain states should be a big indicator. If an individual obtains marihuana legally in a state that it is legal then the cops have no right to bust them for anything. They are not breaking any laws.
No, I don't believe police should be allowed to stop cars suspected of carrying marijuana from a state in which it was legally purchased. Police should stop people who have broken a law. If they begin to stop people they just suspect of doing something, police will be stopping every car that crosses state lines. This is not a good use of police resources.