While I would want this, it's still possible to trade this furniture via housing placement which would dip into the sales of these items. By doing nothing they not only limit the stock of items that can be shuffeled around in-game, but it encourage people to buy more retainers. They would need to tether the furniture to the purchaser and a disconnect of that tether (attempted removal by a 3rd party or tenant dismissal of the purchaser) would result in the destruction of that item. Which I think is a fair trade for unlimited personal access to furniture you've unlocked, but it's also yet another process that would need to be wedged into the code. Overall they get more profit from doing nothing.