It makes sense that you would need a star. Otherwise, it isn't really a system, just an empty region of space. You would get no heat what-so-ever on the planet without a star in the system. This would relegate all planets, including breatheables, to domes. As breatheables can't have domes, it is easier to disallow creating planets in systems with no star.
Also, you may well need the gravitational eddies that result from having a star in the system in order to use the planet creating devices to create stable planets.
Of course, this is not entirely based on real science, just trying to postulate the reasoning behind requiring a star to build a planet.
