>Well, the simple answer is that the farmers have no need to buy the MON seed.<
Unless, through no fault of their own, their crop becomes contaminated with Monsanto seeds. Then they enter litigation hell against a company with deep pockets. This is the more pernicious angle.
>But seriously, how is this different from a software license?<
Except that you can use that piece of software with the computer for as long as you wish.
If we draw a parallel to Monsanto's case, you'd have to burn your disk after one year of use.