The Classic schemes are played a lot in the Classic League. Cups have to include the less popular schemes because of the obvious failure of the Free League, not to mention that those are just as fun as any other schemes. You are not obligated to join.
Although, I keep saying that the points and ranks for Cups should be removed. There is no point, the won Cups should be shown on the player's profile and also on the clan page, nothing else is needed I think.