It is a myth that farmers who lease land next to GMO fields have to sign any agreement stipulating they will not grow corn. When we grow crops for seed production, it is our responsibility to ensure that we have the appropriate buffers to allow for production of high-quality seed.
Regarding your second question, “Isn’t cross contamination a myth,” cross pollination of crops can and does happen. Steve Savage addressed this question in a separate response. Following is an excerpt from his response.
You can view his complete response here: http://gmoanswers.com/ask/how-can-companies-producing-gmo-plants-confirm-their-plants-will-not-affect-non-gmo-plants-could.
“GMO versions of a crop can cross pollinate non-GMO versions of the same crop, but this is nothing new to agriculture. For a very long time it has been necessary to isolate seed production fields of various crops so that the seed will be of the pure, desired type. The size of the buffer needed is something well worked-out depending on the crop and how it is pollinated (self-fertilization, wind, insects, birds...). GMO and non-GMO seed production can be managed in the same way with regard to this issue. All of these issues are very familiar to botanists, seed producers and others in agriculture.”