So there is a little misunderstanding here in that the licensee who posted the material is allowed to share their materials under any license they want, wherever they want and they do not have to use the same license all the time.
If they want to post material here under CC0 they can.
If they want to post materials on their own site with stricter licenses they can.
If they want to sell the material to an individual with a custom license they can.
That is all fine. You just have to make sure that YOU, as the user, are using it under the license that you access it under. The creater holds the copyright to the material and they can license it however they want to whomever they want and the license can be different.
So there is a little misunderstanding here in that the licensee who posted the material is allowed to share their materials under any license they want, wherever they want and they do not have to use the same license all the time.
If they want to post material here under CC0 they can.
If they want to post materials on their own site with stricter licenses they can.
If they want to sell the material to an individual with a custom license they can.
That is all fine. You just have to make sure that YOU, as the user, are using it under the license that you access it under. The creater holds the copyright to the material and they can license it however they want to whomever they want and the license can be different.