While some of its members may genuinely believe they are doing good by their fellow human beings in protecting health and potentially dangerous things, as they think genetically modified plants to be, the dangers of the Precautionary Principle are highlighted to a stark degree by the activities of Greenpeace activists in Canberra, Australia. According to a report, trials in producing GM wheat have been badly damaged.
The persons who did this will, hopefully, be caught and punished with the full weight of the law. Remember, if these guys had their way, the Agricultural Revolution that took place in the decades leading up to the Industrial Revolution might not have happened, or at least to the same degree.
Here is an article by the excellent Ronald Bailey on the GM crops issue.