Well, just because in the canon Star Wars universe they were never able to build more than the prototype for the Death Star - doesn't mean that in the Star Wars Space Empires universe the Rebellion will be just as successful at preventing their deployment, so you keep it as a valid option for later in the game...perhaps those Imperial engineers were able to come up with a fully functional Death Star. Plus, couldn't the Death Star prototype still move around in order to reach the different planets that it would target? I don't see the issue really.
