ZIP is built-in to Mac OS X now, and has been for quite some time. You can unzip a file by double-clicking it, and zip a file by right-clicking and selecting "Create Archive of..."
You can download StuffIt Expander free from http://www.allume.com. It seems Apple is moving away from the proprietary .sit format toward a more universal archive format like .zip.
It's not a factor of what machines come with StuffIt, it's a matter of what operating systems come with StuffIt -- it has nothing to do with the MacBook vs. the iBook G4.