If you look Modules/mathmodule.c you will notice there is a comment that goes with erf() and erfc() stating that the algorithms were taken from a book entitled 'Numerical Recipes'. Unfortunately that book has a license dictating that any information from the book is only allowed for non-commercial use; commercial use requires negotiating a license (http://numerical.recipes/aboutNR3license.html). That's bad for anyone who has a commercial distribution of Python as that's a special requirement they have to follow.

It would be best to do a clean room implementation of both math.erf() and math.erfc() that does not use information from 'Numerical Recipes' in order to not be violating that license. That way Python can be sold commercially without having to negotiate a separate license just for those two functions.

Unfortunately this code exists since at least Python 2.7, so I have flagged all Python releases as needing the eventual clean room implementation applied to it (although Python 3.2 goes out of security maintenance next month so I don't know how critical it is to fix that far back).