You have that backwards OP. Setting furniture in the ground is the default for the game engine, they would have needed to create a different function to allow table top items but this would have all been done before the game was released. Making new furniture into a tabletop item or to make it a table top would be as simple as changing a couple modifiers in the database. Every furniture they release they deliberately choose which one it will be and for whatever reason, this is how they made them.