Why doesn't Mac OS X source ~/.bashrc?
In OSX the terminal  by default starts a login session so reads .bash_profile etc. (The GUI login process that asks for your name and password does not use shell scripts and starts no shell it is all done from launchd and the workspace)
On other Unices xterm runs a non login shell by default so they read .bashrc as the scripts that present you with your password etc at login call the login session and all terminals are sub process of this and inherit the shell environment.
From the GNU document you referred to

Invoked as an interactive non-login shell
When an interactive shell that is not a login shell is started, Bash
reads and executes commands from ~/.bashrc, if that file exists. This
may be inhibited by using the --norc option. The --rcfile file option
will force Bash to read and execute commands from file instead of
~/.bashrc.
So, typically, your ~/.bash_profile contains the line
if [ -f ~/.bashrc ]; then . ~/.bashrc; fi 

after (or before) any
login-specific initializations.