Why not leave "booby traps"?
The Borg want to assimilate everybody for their own good. Their zombie Drones are ill equipped to catch most of these stupid people who don't realize they ought to submit to assimilation. Seeding the entire universe with nanoprobes preemptively sounds like a prudent approach, then.
The victims of the Borg are not a threat the Borg need to deal with. They are targets of charity. Not taking every available measure to help the poor saps means letting the charity work fail. (Which I guess is permissible in charity, which is voluntary stuff not regulated by anything much, even if inexcusable in a government welfare program. But the Borg approach might actually be closer to the latter.)
Timo Saloniemi