Is saving a humans life not worth the few dollars it would cost each tax payer? Those fortunate enough to be able to afford this have a duty to help those in need, regardless of how reckless their actions may have been. You can't just say "I dont care, and only those who care should pay". The few that would pay would need to contribute massive amount of funds. When everyone contributes, the price required from each individual is trivial. We aren't simply alone in our (repsected) countries, we are a part of a larger society that lives there, and thus we have a responsibility to contribute back to the community that has provided the opportunities for us in the first place.
Besides, lower levels of HIV/Hep benefits the whole of society, not just the drug users. After all, sharing needles is not the only way HIV/Hep is spread - its not only the drug users that are at risk of it.