Isn't this illegal?
Not yet.  The Oracle v. Google case hasn't been decided yet, and APIs are
traditionally ineligible for copyright protection in the U.S. and most western
countries.  The MEAD project will purposely not infringe on any trademarked or
copyrighted materials from Apple or any of their licensors; think of the situation
between CEntOS and RedHat--CEntOS is literally a binary recompilation of the RedHat
open source code but stripped of all logos, artwork, etcetera.  Barring a disastrous
outcome in the abovementioned case (which would only apply in the U.S.), MEAD is in
the free and clear as we're not even bothering with any non-API code.