The idea is that large, complicated indoor areas, like malls, airports, casinos, and public transit hubs, would be mapped the same way that cities are. It's a lot trickier than outdoor mapping; your location needs to be much more precise, along with which direction you're facing, because the hallways and rooms are smaller and more intricate per square foot than large cities. If your phone is off by half a block in a city, that's okay; you can still figure out where you are. If you're wandering through a casino and your phone is off by that much? You might as well not be using it.