Say this treatment is done to you, yes you now have HIV, but the HIV you have is not the same HIV Magic Johnson has. Magic Johnson's HIV attacks his T cells and destroys his immune system, your genetically modified HIV attacks cancer cells and leaves your healthy T cells alone.
Simple:
Yes you are injected with a virus, but that virus is good for you.
It's the same idea that Botix is derived from a deadly toxin, but people directly injected into themselves for appearances and does not harm them (if done correctly)