Even if they are sued and lose, the fine will most likely be a percentage point of their operating profits.
Essentially the drift is a positive for them. Neighboring crop fields are damaged. The farmers cannot afford to sue Monsanto or fear doing so, and their only recourse is to switch over to Monsanto branded seeds next season.
Another interesting tactic is that if you plant a Monsanto crop for a season, you are not allowed to use any seeds generated by that crop to plant the next season, you have to purchase new seeds every season. [Edit: This next point was debunked] In addition if any seeds blow onto a neighbors farm and those seeds grow, that farmer can and will be sued by Monsanto for growing their IP.