OK, I take your point that giving the good guys the ability to prove they have only been flying where they should be probably wouldn't need much more in the way of infrastructure than currently exists.
But it's not the good guys we should be worrying about. Monitoring where the bad guys fly (bearing in mind that they may have hacked the firmware and/or geofencing, and not be reliant at all on manufacturer updates) is likely to require a whole new approach to system architecture and infrastructure.
I stand by my view that there would be a significant cost attached to that.