People have differing ideas about what acceptable and unacceptable behavior is. If we required that taxes only be collected for the set of behaviors everyone agreed on, we'd have no taxes, and no government.
Even if we agreed that promiscuous people should not receive public medical treatment (which is not a position I hold), would you suggest condemning people who contracted this through no act of their own as well?
Parents can give it to their children, and there are nonsexual ways you can end up with bodily fluid transmission.
Assuming you're describing the book I think you are, there are a number of editions of said book in which one of the protagonists shows mercy toward the sick and ostracized people.