As long as there's a clear UI separation, there's no harm in providing both first and third party servers for brand perception & user experience. A similar issue is content provenance. In StarCraft II, for example, official maps and custom maps live in separate tabs. No one expects a Dragon Ball-themed map to be made with the same standard as an official Blizzard map.

I suspect it's simply to control monetization and without the extra engineering cost of private server releases.