The answer here is "monsanto is a de facto monopoly" - they're essentially the only company on earth that this kind of business can be done with on any kind of scale.

A decent intro to Monsanto (now Bayer, really) that covers some elements of their monopolistic behaviour: https://www.ethicalconsumer.org/home-garden/monsanto

tl;dr, there is a monopoly, and america decides if that monopoly can do business with Cuba