From my understanding, if you purchase Monsanto seeds you explicitly agree to not reuse seed from your crop. You have to purchase new seed from Monsanto for the next crop. Which I don't feel I necessarily disagree with that practice. You made the deal so you have to abide by it.

The problem is that Monsanto claims to own the rights to all resulting seeds from their original, forever into the future. This includes cross pollination. So they want farmers who never agreed to use their seed in their crops to pay up because nature does its thing and everyone's crop ends up with characteristics of Monsanto's seed.

The solution is for every farmer to pay Monsanto or ban nature.