C’est un vieille question que l’on retrouve dans d’autres CMS.
Exemple :
https://wordpress.org/news/2009/07/themes-are-gpl-too/
Pour simplifier, le code HTML et CSS n’a pas besoin d’être en GPL, mais le code PHP qui utilise l’API de SPIP doit être fourni avec la licence GPL.
Extrait de la FAQ GPL : http://www.gnu.org/licenses/gpl-faq.html#GPLAndPlugins
"It depends on how the program invokes its plug-ins. If the program uses fork and exec to invoke plug-ins, then the plug-ins are separate programs, so the license for the main program makes no requirements for them.
If the program dynamically links plug-ins, and they make function calls to each other and share data structures, we believe they form a single program, which must be treated as an extension of both the main program and the plug-ins. This means you must license the plug-in under the GPL or a GPL-compatible free software license and distribute it with source code in a GPL-compliant way.
If the program dynamically links plug-ins, but the communication between them is limited to invoking the ‘main’ function of the plug-in with some options and waiting for it to return, that is a borderline case."
