It doesn't look so easy. First you must put a GPS in each device, I think the cheaper drones don't have one. Also, the forbidden areas may vary, so they have to provide a way to update it.
A gps costs under $1 today:
http://www.alibaba.com/showroom/gps-chip-price.html
>Also, the forbidden areas may vary
It's mostly airports that we need to worry about, and they don't tend to change much. Sure, TMA boundaries might vary occasionally, but that's not really much of a worry, because most drones won't be up at 1000+ ft.
At DEFCON someone demonstrated dropping drones from the sky by spoofing GPS coordinates to inside an airport. Funny, but it guarantees I won't be buying one of them because of the potential for harm to my camera, etc.
They're also prohibitively heavy on smaller models. A 10% increase in mass (for a 300g quadrotor) can shave an entire minute off of your run time.
The cheaper ones are frequently controlled by a smartphone app, which can do the geofencing for it.
More useful would be a lower ceiling in no-fly zones, tbh; no-fly is appropriate for real aircraft, but for toys there is value in considering "no-fly" and "low-fly".