don't worry, they have their reasons. You don't buy them straight from the OEM, which is ATI. They are sold to laptop manufacturer companies like HP, Dell or Toshiba which have their own lines of support.
Same way, ATI sends its drivers to those companies as well, and they provide it to you. If ATI started supporting mobility chipsets you would have a lot of problems with your laptop's manufacturer because they would not support anything that doesn't have their own drivers on the system.