It doesn't *have* to be done. There's a gigantic number of seeds which are commercially available already, there's many government and private organizations safeguarding these seeds, and the amount of patented seeds is comparatively insignificant. In addition, modern farming operations don't save seeds for future crops.
In fact, the basic idea behind Monsanto controlling the seed supply has been standard industry practice for 50+ years. Most vegetables commercially grown are F1 hybrids. In 1960, 99% of corn grown was an F1 hybrid. If you buy a Better Boy Tomato, an F1 hybrid also popular for home gardening, there will be little variation between the plants grown. However, the seed from these plants will be unusable. Peas and beans pollinate their own flowers, so for these plants such a strategy isn't practical. However, that doesn't mean the death of the species - even if most commercially grown tomatoes are F1 hybrids where the seeds are unusable, of course there's still a million variety of tomatoes which may be planted from seed (with a little care) and are easily available.
This is a symbolic marketing/propaganda move against Monsanto. Monsanto developed a soybean that is invulnerable to the safe, cheap, and environmentally benign herbicide Round-Up. They sell seeds with a contract stipulation that the seeds not be re-sown (again, normal farming practice is to buy all seed anyway), and won a lawsuit against a farmer who intentionally grow a Monsanto crop without paying Monsanto- he would buy the Monsanto crop for the first planting of the year, and use saved seed for the second planting of the year.
This group imagines that it's hard to find seeds that aren't patented, or at least that it will be in the future, to make some point that you shouldn't be allowed to patent seeds because think of how horrible it would be if you needed to deal with Monsanto to plant a carrot. However, if that future does come to pass, this wouldn't really help - you'd need something with the infrastructure to supply huge amounts of seeds, not just supply fun little seed packets to home gardeners.