[caf]

Nobody forced him to use Monsanto's seeds

It's not so black-and-white as this. You can typically buy excess seeds from the local grain elevator for replanting; these now contain an unseperable proportion of the descendants of Monsanto's seeds, so it's at least become lot harder not to use any of them.

In other cases (not this one), the farmer's field has been cross-contaminated from nearby fields of GMO soy, so they have been forced to use the descendants of Monsanto seed. Are these cases different? Why, in patent law? (Patent infringement does not require intent).

The environment that Monsanto are trying to create is one in which you must pay a royalty to Monsanto if your crop contains any of the descendants of their seeds, and where completely avoiding their seeds is impossible or at least very difficult (and imposes new costs).