Not every person taking the medication has been infected with the disease. There have been numerous cases of flesh eating disease in the US, most notably the young girl who lost limbs.
The articles have stated the drug along w chemotherapies caused a weakened immune system which kinda created the perfect storm. The patients had internal bleeding, open wounds which allowed for entry by staphylococcus pyogenes. I guess the alternative for these patients would of been death because of refusing treatment, instead of having a chance by trying them. And its important to remember, this isnt everyone taking the medicine- just a link that has been found (along w chemotherapy)