I am vaguely quoting open-source advocates.
It is in the actual license of those assets - you gain access to their sources and you are allowed to share them under Creative Commons license. You could think of it as a next step beyond DRM-free.
Specifics can vary - CC licenses can range from indistinguishable from public domain, through you-can-modify-only-if-you-share to fairly restrictive share-don't-modify.
Some want you to credit the author, some don't. But - most importantly - there is nothing about preventing author to sell their work with that kind of license.
So the license makes it free as in free-speech - you can share it as much as you like, not as in free-beer - that you are entitled to receive the thing.