Wordpress под своей лицензией, плагины под своей.
Actually, that’s the same with all plugins and themes on WordPress if you read the WordPress fine-print. Because WordPress use’s the GPL_v2 licence, any plugin or theme that is released for it must also be GPL_v2 compliant.
So the only thing you can actually charge for is the original code and then the service of updating.
Being a programmer and not a lawyer, I don’t know much about licensing.
All I ask is that ACF PRO is not distributed within a free plugin / theme.
Otherwise, it would not be possible for me to work full time on this project.