I know the lore is kind of confusing but it works like this.

A mortal must either willingly give a vampire their blood in some way or the vampire must have a good reason for taking blood (such as being close to starvation or death). While there aren't any penalties for taking blood for no reason and without consent, it is viewed as abhorrent to almost all undead, forcing those who do this to live in remote places far from other unlike minded vampires.

It's more like a rule that was created thanks to the hunts in that kingdom [@The Wyrm] mentioned in his characters bio as well as several vampire hunts that took place in the distant past.