First of all, soybeans are largely self pollinating; that is, the pollen transfers from the stamen to the pistil before the bean blossoms. While some pollen can become windborne, and fertilize the pistil of a different plant, it's the exeception, not the rule.
But, let's assume that the chances were 50-50 that the bean would be pollinated by a different plant and let's ignore that the likelyhood of one plant being pollinated by another decreases geometrically with distance, the odds of an advantageous trait accidentally transferring from one field to another would be so low as to be mathmatically impossible. (keep in mind that the Round Up ready trait provides no benefit unless it is found in virtually all beans in the same field).
There are about 180,000 soybeans in a bushel, an average yield would be about 50 bushels per acre and a typical field maybe 40 acres. So, 360,000,000 soybeans would be a fair test population.
Just as heads wouldn't innocently turn up each time in 360,000,000 coin flips, virtually all of 360,000,000 million soybeans wouldn't be cross pollinated by a neighbor's field.
And yet it's because of this possibility, exceptional though it might be, that the big companies are able to intimidate and extort the small farmers. But it needn't simply be soybeans: it could be corn, apples, heck anything.
But, let's assume that the chances were 50-50 [...]
You are concentrating on one thing: stupid soybeans.
That is not the issue at hand; the issue is: these companies are laying a [legal] groundwork to seize control of food production at the lowest levels. -- Indeed to even make traditional [heirloom] plantings a liability.