The terminator gene solves the gene-spreading problem, but it introduces the problem of leaving farmers permanently at the hands of Monsanto. They are forced to buy new seeds every year.
They can, of course, opt out, but then they miss out on Monsanto's improvements. So we've got a conflict of expectations not entirely unlike Slashdot's frequent outrage about EULAs that effectively mean you don't own your own software, or even hardware.
As I understand it, most farmers buy seeds anyway, because the plants don't breed true to type. But there was particular worry about poor nations, where the farmers are closer to being completely broke, and this looked suspiciously like indentured servitude.
I'm not taking a position on the argument here, just clarifying what it's about.