The argument is that changes to genetics bridge generations. If you kill someone with medicine they die. If you give someone a faulty gene, it could affect exponential number of people with each generation.

Unless it affects reproduction ;)

It is the same with antibiotics, people that naturally would not be able to reproduce, will, so the global genetic outcome will change, and possibly something making a given person susceptible to a given infection will be passed to the next generation. But nobody, fortunately, pretends that it is a bad idea to give antibiotics to patients.