You think? Why do a lot of D3 Bots still work and not get banned instantly? At least one of them uses such an overlay and Blizzard has not used your "easy" concept of calling the overlay API.
Trust me, if you challenge some people they will beat you until you sit crying in a corner
because they have the brain and the dedication.
Privacy concerns often trump IP concerns. At least in the EU.
And again people only would get back what they already had and was considered fair game. Blizzard will only look bad.