"CUPS is provided under the GNU General Public License ("GPL") and GNU Library General Public License ("LGPL"), Version 2, with an exception for Apple operating systems. A copy of the exception and licenses follow this introduction."
EDIT: Just to provide the full exception clause text.
"Apple Operating System Development License Exception;
Software that is developed by any person or entity for an Apple Operating System ("Apple OS-Developed Software"), including but not limited to Apple and third party printer drivers, filters, and backends for an Apple Operating System, that is linked to the CUPS imaging library or based on any sample filters or backends provided with CUPS shall not be considered to be a derivative work or collective work based on the CUPS program and is exempt from the mandatory source code release clauses of the GNU GPL. You may therefore distribute linked combinations of the CUPS imaging library with Apple OS-Developed Software without releasing the source code of the Apple OS-Developed Software. You may also use sample filters and backends provided with CUPS to develop Apple OS-Developed Software without releasing the source code of the Apple OS-Developed Software.
An Apple Operating System means any operating system software developed and/or marketed by Apple Inc., including but not limited to all existing releases and versions of Apple's Darwin, OS X, and OS X Server products and all follow-on releases and future versions thereof.
This exception is only available for Apple OS-Developed Software and does not apply to software that is distributed for use on other operating systems.
All CUPS software that falls under this license exception have the following text at the top of each source file:
This file is subject to the Apple OS-Developed Software exception."
pyre already acknowledged that Apple now owns the code. The reason why CUPS is GPL+/LGPL+the Apple OS exception? Because Apple bought the rights to it and made it that way. Before that, it was GPL/LGPL.
"CUPS is provided under the GNU General Public License ("GPL") and GNU Library General Public License ("LGPL"), Version 2, with an exception for Apple operating systems. A copy of the exception and licenses follow this introduction."
https://www.cups.org/documentation.php/doc-2.0/license.html
EDIT: Just to provide the full exception clause text.
"Apple Operating System Development License Exception;
Software that is developed by any person or entity for an Apple Operating System ("Apple OS-Developed Software"), including but not limited to Apple and third party printer drivers, filters, and backends for an Apple Operating System, that is linked to the CUPS imaging library or based on any sample filters or backends provided with CUPS shall not be considered to be a derivative work or collective work based on the CUPS program and is exempt from the mandatory source code release clauses of the GNU GPL. You may therefore distribute linked combinations of the CUPS imaging library with Apple OS-Developed Software without releasing the source code of the Apple OS-Developed Software. You may also use sample filters and backends provided with CUPS to develop Apple OS-Developed Software without releasing the source code of the Apple OS-Developed Software.
An Apple Operating System means any operating system software developed and/or marketed by Apple Inc., including but not limited to all existing releases and versions of Apple's Darwin, OS X, and OS X Server products and all follow-on releases and future versions thereof.
This exception is only available for Apple OS-Developed Software and does not apply to software that is distributed for use on other operating systems. All CUPS software that falls under this license exception have the following text at the top of each source file: This file is subject to the Apple OS-Developed Software exception."