If they forced the maintainer of some FOSS software to merge in some code, even if the maintainer isn't even allowed to speak about it eventually someone would notice (since open source), fork the project and just cherry pick out their crap. Then it's whack-a-mole of trying to keep people from multiplying it.
Or they could claim the software is illegal and have no way to enforce that either.
So basically as long as said software is useful for more than a handful of people, it's infeasible to try to enforce it (e.g. see how it goes every time some software gets a cease and desist, they end up even more popular than before)