Well, you are more or less right. OGS was designed for humans, and all the bots are run by third parties and the user friendliness is almost non existent 
However, once you figure it out itĖfine you can usually find the restrictions in the botās profile, or check here: AI Game Restrictions
As to why there restrictions exist (apart from bots designed for certain board size) I think you are quite close with the guess that it is due to hardware limitations, as not to waste āLeela powerā on 20k gamesā¦