I'll give you an example. My brother has a Toshiba Satellite A200 laptop equipped with an ATI Mobility Radeon x1400.
Since its release in 2007, Toshiba does not update the graphics drivers, and I had to manually modify the setup INF files to install a recent version of Catalyst (because some games failed with the original drivers).
Here a clear example of the problem I mention.
The funny thing is that these limitations (in case of Mobility) are "artificial" because, as I say, simply a change in the driver installation files allow installation on any computer (you can also use the Omega Drivers, which are already modified).
I understand what you say, but I think it would not hurt to have this other option, besides being extremely easy for the manufacturer (AMD).
I guess this will be due to trade agreements.
By the way, sorry for my English