Your reasonsing about why you need the XSkey inserted in your new Mac but not your old one..is correct.... It's an oddity with the licencing system that requires this in certain circumstances.
However if you should ever update to Logic 9 on your new Mac, the requirement for the XSkey goes away.. as the version of Logic 9 from the App Store, is the full version and doesn't rely on any previous versions to install... though you must remove the Logic Studio System and/or ProApps ID files usually found in /Library/Application Support/ProApps, prior to install of the App Store Logic 9, for that to be true in every case... oyherwise it will ask for prior serial numbers to complete installation oddly enough.
I supsect all these odd behaviors will vanish upon the next major release of Logic as then everyone will be installing a full version and no one will be using upgrade versions of Logic Pro like they still can with Logic Pro 9 or earlier... if they own the boxed sets...