Because he bought seeds he knew were transgenic
He bought generic seeds that may or may not have been transgenic. (He may have guessed that they were likely to be, but he didn't "know.") They weren't sold as "transgenic," they were sold as "soybean seeds!"
What's happened here is that what should have been a perfectly normal sale (as humans have done for thousands of years) of perfectly normal seeds (as have been grown and saved also for thousands of years) has somehow become beholden to monopolistic amoral corporation, and that the system (legal, economics, USDA regulations, etc.) makes it exceedingly difficult to avoid it.
What is happening here is both wrong and dangerous on many levels: food safety, property rights (such as the right of a farmer to use his own seeds), etc.