The difference is that if mere propagation were problematic, then Monsanto directly harmed him by the contamination. He could use the "unclean hands" defense if Monsanto sued. But if he took deliberate action to concentrate the genes, over and above what would normally be done just growing crops, that defense would no longer be available.

Monsanto's statements that it would not sue for just contamination can be viewed as immunizing them against being sued for that contamination.