The moral issue is that Scarlett Johansson's voice is famous and to many people the fact that it sounds like her specifically is the source of much of the value

The moral issue is that Scarlett Johansson's voice is famous and to many people the fact that it sounds like her specifically is the source of much of the value of that voice, so one can argue that the benefit of that should accrue to her, not to an unknown person who happens to (or can make herself) sound like her or to the company she refused to allow to use her voice. The legal issue is that there are specific, established laws based on ideas like the above, known as "right to publicity" laws.