Mac OS X comes with Ruby already installed, and afaik you don't need a C compiler to learn standard ruby, you'd only need it for certain gems (which want to build native extensions).
OS X comes with an obsolete version of Ruby which almost nobody uses.
The latest version of OS X comes with Ruby 2.0, which is hardly obsolete, and that or Ruby 1.8.7 (from earlier OS X) is totally fine for learning to use Ruby, which is how this thread started.
Claiming you need to start to learn by installing xCode or the tools is false - there is lots more to ruby than installing rvm, rails and sql gems etc. Beginners could go a long way without requiring non-native gems, and by the time they get to that stage, installing Ruby 2.0 should be a breeze, whatever route you choose.
Probably over 95% of mac users would never use the compiler, so I see why they left it out. Installing it is really very easy anyway, command line or with xcode.
Mac OS X comes with Ruby already installed, and afaik you don't need a C compiler to learn standard ruby, you'd only need it for certain gems (which want to build native extensions).