Companies like Monsanto and DuPont create genetically modified seeds to resit pesticides, which is good for crop yields. But they then use their patent to punish farmers who do not buy their seeds by suing them for using the genetic modifications without a license when their crops have been contaminated by natural cross pollination. They have sued more than 410 farmers to date and are closing the seed processing facilities that farmers depend on. There will be no point to labeling GMOs when they dominate the seed industry. They currently own > 53% already.
Sources:
http://www.huffingtonpost.com/2013/02/13/genetically-modified-seed-paten...
http://www.organicconsumers.org/articles/article_16844.cfm
https://www.commondreams.org/headline/2012/01/31-0