RE: Introducing Praetoria: Rise of Champions

You can be in gold that allow 1 champions, but for any reason you don't have enough to build that champion into gold level, so you use bronze level champion for the time being.

Reasons of why bronze allow 0 champion is for simplicity. It's very common for games to introduce complexity as the game progress, and bronze is a very early stage. Too much complexity will overwhelm players.