My understanding is that those are the ones that are tied by the EULA to a particular machine. In return you get a discount on the price of the software.

Maybe Office 2000 had more restrictive terms, but my retail copy of Office 2003 Pro has a EULA with the following:

1.1\tInstallation and use. You may:
\t(a) install and use a copy of the Software on one personal computer or other device; and
\t(b) install an additional copy of the Software on a second, portable device for the exclusive use of the primary user of the first copy of the Software.

...

2.1\tMandatory Activation. THERE ARE TECHNOLOGICAL MEASURES IN THIS SOFTWARE THAT ARE DESIGNED TO PREVENT UNLICENSED USE OF THE SOFTWARE. You may not be able to exercise your rights to the Software under this EULA after a finite number of product launches unless you activate your copy of the Software in the manner described during the launch sequence. You may also need to reactivate the Software if you modify your computer hardware or alter the Software. Microsoft will use those measures to confirm you have a legally licensed copy of the Software. If you are not using a licensed copy of the Software, you are not allowed to install the Software or future Software updates. Microsoft will not collect any personally identifiable information from your device during this process.

...

14.\tSOFTWARE TRANSFER. Internal. You may transfer your copy of the Software to a different device. After the transfer, you must completely remove the Software from the former device.

Note that I transfered my previous retail copy of Office XP to a new machine without incident (only had to reactivate). I've been told that MS resets the activation info every 120 days, so as long as you aren't installing the same Office to new machines more often than that (like a sudden 20 installs of the same copy) they don't care so much. If this same EULA was in effect then (and I believe it was), I was OK due to item 14.