But if his fields had been naturally pollinated, why should he be responsible for Monsanto's inability to contain their pollen?
Because he specifically decided to replant the Monsanto seed. It's one thing to have your crops polluted; it's quite another to say "hey, I like this pollution and I'm going to spread it further".
In fact, if he was in the business of selling non-GMO, the contamination of his fields could cost him value, customers, or even entire markets.
Which is why Monsanto agreed to pay him for the costs of eliminating the Monsanto seed which had been accidentally blown into his field.