Monsanto doesn't sue for cross pollination, they sue for breach of contract or patent infringement (theft). Even then, there has only been 147 lawsuits filed since 1997. It certainly doesn't happen everyday. If you find Monsanto seed has somehow found its way into your crops, they will remove it for you and pay for it. Also, I don't see any reason why Monsanto would want to get into the cannabis business.
Most farmers do not want to save their seeds anyway. The offspring do not tend to reliably carry the desired traits. It's easier to just buy new seeds. This is true whether the crop is GE or not, and it's been true since the 1930s when hybrid crops became the norm.
