Despite the fact that the CUPS license uses GPL+exceptions language, it is effectively a dual license. There is nothing wrong with using a dual license, but it would be preferable if they made that clearer.
The main ramification of the exception is that authors of printer drivers can release their driver under a GPL incompatible license and Apple could distribute that driver but Linux distributions could not. This situation is no different than if Apple had called their "GPL+extensions" license the "CUPS License".
So Apple is basically maintaining a GPL version and a CUPS License version; using the CUPS version for themselves (so they aren't required to release source code and they can permit GPL incompatible drivers). They are (only) permitted to do this because they are (presumably) the copyright holders of all CUPS software -- and they require that any contributions either assign them copyright or use the "CUPS License".
Hopefully, printer manufacturers are aware that there are many more Linux installations than Macs and will decide that releasing a GPL version of their driver is in their best interest.
TUCA; ie, IANAL