This has been a concern for a while. I hope it doesn't happen and i think the market will be what can prevent it. Nobody wants Monsanto weed so I think they would struggle to sell their product. The danger comes from over regulation of the market that would funnel business to them artificially; a law that requires seeds to come from an "approved distributor", for example.
This does worry me a bit but if it does happen, the market will likely just move back "underground" and all the producers and consumers go back to being outlaws like in the old days.
RE: Mounting evidence BAYER - MONSANTO to control CANNABIS?