What exactly are you proposing? Phillip Morris can't get into any aspect of the marijuana industry? So what's to stop the people that own stock in the tobacco companies from buying stock in marijuana companies? But never mind that obvious and easy work around, I'd imagine with the infrastructure that they have in place already that they could get weed to people much cheaper than most others in the industry, isn't that a good thing for consumers?
I think the primary concern is what kind of product that they may come out with that works to addict their new consumer base.
The high does the trick. Your brain does the rest. They might spend money to make smoking or using weed as more socially acceptable in public. Or breed stronger strains for their more loyal users.