I recently decided to take a look at the spine runtime git repo to see if there were new updates so I can apply the changes to my GameMaker port. I noticed the runtime license was updated with a rather confusing (confusing to me at least) clause:
Without the written permission of Esoteric Software, you may not (a) modify, translate, adapt or otherwise create derivative works, improvements of the Software or develop new applications using the Software
The commit message indicates that permission to create derivative works is included as part of the Spine software license:
Version 2 of the license fixes this by allowing anyone to use the Spine Runtimes but only for internal use. When you purchase Spine, the Spine license (http://esotericsoftware.com/spine/files/license.txt) grants you the right to make and distribute derivative works of the Spine Runtimes.
Maybe I'm just not looking hard enough, but I don't see that permission to create derivative works in the license.txt file.