That doesn't make any sense. An AP is just an AP, it has no internet connection. How does the smartphone get internet connection? With UMTS/LTE or similar. So you still need a data plan (or a combination with it).
The point is that without a way to tether your phone to a 'real computer', you have to be actively trying to use more than a couple of GB of data per month, so "unlimited" really means "up to maybe 5GB". While tethering was possible beforehand, it became much easier when wifi tethering was added to Android. This let anyone push a button and use their 3G / 4G data connection for torrenting, streaming and other such heavy duty usage, and suddenly "unlimited" phone plans were seeing hundreds of GB per month.