The reason is that billing is separate, via the paid tier of the API. Just a few minutes ago, I was able to test Gemini CLI using a Workspace account after setting up a project in the free tier of the API. However, that seems to have been a bug on their end, because I now get 403 errors (Forbidden) with that configuration. The remaining options are either to set up billing for the API or use a non-Workspace Google account.