That would be because we have copyright laws.
We don’t have laws that say you can’t plant seeds as seeds.
Monsanto has contracts that specifically say that, but as I understand it there were no such contracts signed for the purchase of the feed.
So, a farmer could purchase from a local elevator and replant replant corn (almost all corn is hybrid) without violating any patent because the characterististics of the next generation differ substantially from those of the plant which produced the seed. This soybean seed was not hybrid and the farmer purchased it knowing what the traits of the plants would be.