Is it really though? Anymore than soda, coffee, etc.? I don't see any evidence that marijuana legalization would affect their sales. In fact, they often go hand-in-hand, from my experiences in college, so maybe legalization would help their sales.
Personally, I figure Phillip Morris will be all over marijuana as soon as it is legal at a federal level. Tobacco is slowly being stomped out by ever more stringent restrictions and regulations, so marijuana could even be the future of their businesses as tobacco slowly sinks into obscurity.
They already have farms, equipment, and sales/distribution experience too. Sure, there is a lot of difference between the two products, but they would have to be pretty incompetent to be completely unable to leverage their existing assets to dominate the pot market.
They purchased 100,000+ acres of Humboldt county in the early nineties. Not sure if the kept it, considering that premium cannibis is now grown indoors.