Roundup will kill all non-monsanto plants; they change the formula every year to make sure that you have to keep buying seeds. In this case, the farmer grabbed a ton of seeds, sprayed everything with roundup and kept only the seeds which survived; the seeds he knew to be from Monsanto.
I don't like the idea that Monsanto can go to your farm and throw seeds and suddenly you owe them money, but it seems like they might be in the right here. The farmer did go out of his way to pursue survivable seeds without paying Monsanto for the privilege.
This is different from Yogurt where you have some natural selection. He didn't introduce a chemical to the Yogurt with the intention of killing all non-chemically-protected strains.