Technically it probably has to do with capacity planning, so you have one sat overhead that can serve X number of people, so you sell that number of dishes to that area. With people moving, you have to leave X number of spots unsold as a buffer for people driving into an area or degrade everyone's service.

I believe this has already been a problem with people buying a dish in an available area only to use it in an area that is 'sold out'.

Also it's another opportunity to make more money.