This is actually a strategic move. When a player stops the Charleston, it usually means they’re close to Mah Jongg (typically with fewer than three tiles left to pass). Most experienced players won’t offer any tiles during the courtesy pass in that situation, since they don’t want to help that player complete their hand. 

Our level 3 bots follow this advanced strategy, while the lower level bots don’t, since it’s more of a higher-level play. 

So, if you stop the Charleston, the level 3 bots won't pass tiles during the Courtesy.