Currently there are plenty of Sovereigns in the world just hanging out. Players do not need to spend $100. They could accumulate Sovereigns, over time, from other players, for gold.
Now when "placing" the house you still have to confirm the placement however once it is placed it then acts like any ordinary house. We did not want to edit base code when creating this item. As UO keeps releasing the new Castle and Keep designs we will continue to add those to the ordinary house placement tool. This isn't an item people "need" and with unlimited housing storage right now it is not even an issue.
We also are not sure why UO does not allow these on production shards. Maybe a loading issue? That being said we also didn't want a bunch of these bought up and placed all over Malas right away.