A league who consists only of BOT players does not cost a lot of processing time.
It is only virtually created.
Meaning, the league and the status of the teams at that league is loaded only when the first real player joins it.
that would make sense but that is wrong, bots playing normal game there gs is calculated, they got injurys and so on - and they even interact with normal user when they are in bot only league through cup.
I guess that in most countries there are more than a single player who joins BB in a single week.
Due to that, you can add them simultaneusly (up to 6 players that will get the 6 lowest places) to a single league.
most is not general you know.
BTW, when I joined BB two and "not much" seasons ago there where maybe 3 "real" teams in my league.
So, I think that you may be wrong if you believe that in lower leagues you don't have a lot of BOTs.
but the system fills normally first the league, before it put the players in a new one, so that you normally had some real competion, and interaction to stay at the game.