the licensing i have read meant that if buying an OEM copy it is only to be installed on a single system and can not be moved, (OEM being the $130 dollar version with nothing fancy with it) Retail being he $300 version with all the nice packaging.
A retail copy however can be moved to any other system , as you said, as long as it is removed from the prior system
This is why you can not take say a Dell install Disk and use the key on another system, because it is licensed for installtion solely on that dell.
OEM and Retail disk use a different "code" base so to speak., an OEM code will not work with a Retail disk and vice versa, this is how MS knows if it is an OEM install or a Retail install.