If you treat the drug problem without addressing the cost of the treatment, you may end up doing too good a job. The Hep C treatment could become less profitable for drug companies, driving prices up for the (smaller number of) people who still get Hep C.

You need to address both issues. By your own admission, "Hep C kills more people in the United States than all other infectious diseases combined." It would be outright unethical to do nothing to address treatment for people who already have the disease. And besides, sharing needles doesn't cause Hep C infections: sharing needles with someone with Hep C does.