EA decided that autonomous biting would be too much calamity for a town.
Also, you can reverse the spell as well by making them the normal humans. They even give moodlets like flirty and happy. It can sometimes be fascinating to see these immortal beings using their powers to suck the blood out of a devil or an innocent person, per se.
It makes it so that the sun no longer hurts your vampire sims while they’re out in it.
