While Monsanto does sell seed under the Dekalb brand, they also license to the technology to other seed companies, so it is not simply a matter of buying Monsanto seeds.

Additionally, I don't have my contract with me at the moment, but I seem to recall the requirement of planting a percentage of non-GM crops as part of the agreement of using said seed. Planting RR crops year after year on the same ground doesn't work very well.