1) In a market economy, monopolies should generally be distrusted and prevented whenever possible.
2) There are people who are unsatisfied with the claimed (lack of) health effects of GMO on human consumers.
3) There are people who are unsatisfied with the claimed (lack of) effects that GMO seeds could have on the broader ecosystem.
You shouldn't just disband monopolies too, you should analyze the environment that let them form in the first place. In a functioning market the only time a monopoly becomes an issue is if they can exploit their position to keep prices above optimal competitive levels, and that means that there are barriers to market entry that need to be considered in policy that prevent price competition against the major player. And usually they are legal in nature, be it patent / copyright, or biased tax law, or blatant exclusive contracts like ISPs.