You can sign away a right though. At least over here. Monsanto would have every right to say "sign this contract if you want to buy our seeds, otherwise we're not selling them to you" and said contract including a provision that you won't salvage seeds.
That is the whole problem, Monsanto has a near monopoly on GMO crop seeds that farmers use, their is no viable alternative. As noted before, Monsanto seems to have the money and political power to write laws and get them enacted that protect their patents, forbid truth in labeling, and generally continue practices that are anti-consumer.
The one case that made it to the Supreme Court was for a farmer who went and got seed from where grains were stored. technically, he felt that since he was not replanting his own seed that he had saved he was not violating the contract. Monsanto and the Supreme Court said it did not matter, he was still violating their patent. Thus Monsanto tightened their hold on all of their product, even if it is second or third generation crop seed that came from another source. Monsanto patent prevails forever apparently. Sucks.