Well, one problem is that taking Sabrina up on her offer will almost inevitably make everyone in the city effectively beholden to Sabrina, even dependent on her, for their continued survival. Sure, the first one/two/three are free, but what happens three, six months from now when Sabrina wants something from Tokyo? Will they be able to say no, if their food supply is being threatened?Yes.
I really can not understand why getting your seeds cleansed is not enough and people seem to think it would be better to make us stop cleanse other peoples seeds.... when we explained to them its not an economy of scarcity anymore.
This might not even be wrong; speaking as part of Sabrina's head, I can say pretty definitively that we're a bunch of busybodies, and probably won't be able to stop ourselves from intervening in something we see as wrong. Let's say the city of Nagoya is in the middle of a civil war involving dozens of meguca, which has the potential to kill thousands of innocent civilians. Will we be able to keep our noses out of it, or avoid pressuring reluctant Tokyo girls into helping intervene?
Also, anyone who knows the Witchbomb will realize that what Sabrina is offering is the end of natural selection as a way to keep the meguca population down, which means there's about to be a baby boom of magical girls, in particular the kind who are unwilling or unable to hunt enough Witches of their own to survive (like for example Ono). In six months, the meguca population of Tokyo could easily triple, even more maybe, and all with new girls who have never had to fight a Witch in their lives, which is sure to complicate what's likely already a delicate political balance. Can we trust these new girls to not be frivolous and self-entitled about their powers, to not get into fights just because, hey, they have superpowers now and they deserve to be in charge?
TLDR: There's actually a few rather important reasons that meguca groups would be skeptical about a girl offering infinite free cleanses, even beyond the whole "is this really possible, or just a scam" consideration.