Let the user decide wich level of the cards he will begin to post. The user story map usually initiates with the third level, and the "mapper" organizes it with the upper levels of cards. In this tool, you being with the first level and goes down to the user stories cards.
All my votes for this!
Way to strict on where you can put cards. Do not pander to your customer base when your customer base are IT specialists. Product was much much better when it was effectively just cards on a grid you could drag and drop.