unless those buying the seeds sign a contract which clearly states they assume all responsibility for what the seeds do to their environment
Well, I might not have the same perspective on "muh freedom", but you shouldn't be allowed to sign such a contract at all, because the scope obviously surpasses you. In an ideal world with an ideal justice system, such a contract should be void and both those who sold and those who used the seeds are responsible for the damage.
Sterile seeds have little to do with that, by the way, as they have been easy to produce and have been used for a long time already (sterility can be either desired or undesired depending on the crop, but usually it's just a side effect from hybridisation).
"When the going gets tough, the tough get empirical." -- Jon Carroll