It's to allow games to happen at all, especially during the end when most people have played their share of games already. Yes, it makes those games different, but really, there's no other solution. Thinkable would be:
- switch to two or three player maps if need be. But how do you score them?
- just don't allow a CPU. If you can't get your game going, that's your problem. This solution is just cruel.
- have impartial players stand by to take empty places.
I'm actually thinking about allowing the last option, specifically with me as that player

But I'd prefer a CPU, it shows more consistent performance.
Oh, the server has a forced waiting period. It's not that as soon as three players are together, they can start a game with a CPU. They'll have to wait a bit for a potential fourth player first. I'll adapt the length of that period to the number of players registering.