The installation discs for Mac OS X already contain software licensed under the GNU GPL (e.g. bash), so the additional obligations under the GPL would not have been the reason to exclude a compiler. It seems much more likely that it was the size. Several hundred megabytes that aren't required by most users should always be omitted from the installation discs, so that more room is made for what most users require. The same is done with Ubuntu and Debian installation images. Most of the tools needed to build Ubuntu are not included in the installation image, and require downloading "apt-get build-dep $packagename".