I heard about this too, and it's not surprising, but still..... It's not like California is declaring civil war on the rest of the country or something, so why does the Federal government think they need to step in when a state makes a legitimate legal decision through votes? >_<
I can see one point about how it doesn't help their war against drugs though, as some drug dealer working for a cartel might easily be arrested on suspicion of possessing marijuana, and all the other drugs they have might get confiscated. And so if marijuana is legalized, they can't arrest someone like that so easily. But even still, even if you take out those people and put them in jail or prison, those cartels are just as quick to find new people to do that stuff for them, and the amount of drugs they seize in an arrest like that is just a drop in the bucket of that whole industry. :/
So really, I don't see why they think it's necessary, it must have nothing to do with money since the state could be making more money with it being legalized and unenforced by the Federal government.
IDK though, I'm working with limited economic and legal knowledge, so I'm sure the OP has a better idea of the possible reasons behind it.