If you have a valid Spine license, you are granted rights to use the Spine runtimes in your game, as well as the rights to modify the Spine runtimes to meet your needs.
The issue with cocos2d-x was that they forked the Spine cocos2d-x runtime and put it in cocos2d-x. Their fork quickly became out of date, but also the Spine runtimes require a Spine license to be allowed to use it in your applications. By including it with cocos2d-x, it means all cocos2d-x users need to license Spine! :o