@JWChibi maybe? I mean there could be the reason because the male lead's mana is high his son is able to do all that dark magic. which also explains why the female lead's body doesnt just crumble and such when she is near him. cause he leaks quite an amount to her.
@pocky8 I dmed you the reasoning.
tldr: because they arent tax office. and what they do are not something tax office do, but closer to a bank, or a pension office. and mainly, it also says pension office in korean.