For example, there is no reverse engineered libraries for Facetime and iOS API. If you use Apple's libraries, they can come after you for copyright infringement.
Cappuccino is exactly such a reverse engineered library, as is gnu step. For that matter Linux is a reverse engineered drop in replacement for the unix kernel.
I'm not familiar enough with Cappuccino to comment, but GNU Step was an independent implementation of the published OpenStep interfaces. That isn't reverse engineering.
Linux is an independent implementation of the Posix interfaces.
You can reverse engineer anything that you can reverse engineer. "Supposed" really doesn't come into it. If you buy a copy of osx and reverse engineer some libraries, then publish a description of their behaviour, so what?
That's fine, but if you publish the modified binaries of OS X's libraries with hacks to make them work in Linux along with the description, you will be in legal hot water.
For example, there is no reverse engineered libraries for Facetime and iOS API. If you use Apple's libraries, they can come after you for copyright infringement.