I think the logic there is that the monsters have been mostly forgotten by the general public, so the importance of the Spirit Witches isn't widely known, which means that it's difficult for governments to justify anything they do. Blowing up a perfectly legal establishment for a random slave is not the kind of thing that the kingdom can support. On top of that, Sofia's not the only Spirit Witch, she's just the Spirit Witch of the one kingdom, and if something does happen to her, she can eventually be replaced before the monster problem grows too big.
That said... it doesn't really make that much sense when considering that apparently, this type of monster is extremely difficult to deal with by any other means. I think only that black witch chick ever deals with them, and with great difficulty, using the power of Abadon or something... And even then she had to blow up an entire street or something.
In other words, I get what Wolf was trying to aim for here... but it doesn't actually make sense if you look at it closely, even with the whole "The public isn't fond of Spirit Witches because they're total sluts." thing he tried to establish.