Walter Montego posted about not liking the idea of not being able to challenge people on higher steps on the BrainKing.com board, so I will expand an answer here.

The point of the stairs is that the best players will sooner or later raise to the top steps. If a new person joins a stair, they have to EARN the right to challenge the top player - they need to play people on their own step and slowly climb to earn the right to play the top players.

The purpose of letting people challenge people in the steps right below them is so that will allow the top players to hopefully have someone to challenge so they can at least keep some games going, and not just jump to the top with no games to play. (Then again they do not have to challenge if they do not want to.)